path: root/doc/TODO.detail/vacuum

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From: "Hiroshi Inoue" <Inoue@tpf.co.jp>
To: "Bruce Momjian" <pgman@candle.pha.pa.us>
Cc: "pgsql-hackers" <pgsql-hackers@postgreSQL.org>
Subject: RE: [HACKERS] Index recreation in vacuum
Date: Wed, 19 Jan 2000 10:13:40 +0900
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> -----Original Message-----
> From: Bruce Momjian [mailto:pgman@candle.pha.pa.us]
> 
> [Charset iso-8859-1 unsupported, filtering to ASCII...]
> > Hi all,
> > 
> > I'm trying to implement REINDEX command.
> > 
> > REINDEX operation itself is available everywhere and
> > I've thought about applying it to VACUUM.
> 
> That is a good idea.  Vacuuming of indexes can be very slow.
> 
> > .
> > My plan is as follows.
> > 
> > Add a new option to force index recreation in vacuum
> > and if index recreation is specified.
> 
> Couldn't we auto-recreate indexes based on the number of tuples moved by
> vacuum,

Yes,we could probably do it. But I'm not sure the availability of new
vacuum.

New vacuum would give us a big advantage that
1) Much faster than current if vacuum remove/moves many tuples.
2) Does shrink index files

But in case of abort/crash
1) couldn't choose index scan for the table
2) unique constraints of the table would be lost

I don't know how people estimate this disadvantage.
  
> 
> > Now I'm inclined to use relhasindex of pg_class to
> > validate/invalidate indexes of a table at once.
> 
> There are a few calls to CatalogIndexInsert() that know the 
> system table they
> are using and know it has indexes, so it does not check that field.  You
> could add cases for that.
>

I think there aren't so many places to check.
I would examine it if my idea is OK.

Regards.

Hiroshi Inoue
Inoue@tpf.co.jp

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From: "Hiroshi Inoue" <Inoue@tpf.co.jp>
To: "Bruce Momjian" <pgman@candle.pha.pa.us>
Cc: "pgsql-hackers" <pgsql-hackers@postgreSQL.org>
Subject: RE: [HACKERS] Index recreation in vacuum
Date: Wed, 19 Jan 2000 10:13:40 +0900
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> -----Original Message-----
> From: Bruce Momjian [mailto:pgman@candle.pha.pa.us]
> 
> [Charset iso-8859-1 unsupported, filtering to ASCII...]
> > Hi all,
> > 
> > I'm trying to implement REINDEX command.
> > 
> > REINDEX operation itself is available everywhere and
> > I've thought about applying it to VACUUM.
> 
> That is a good idea.  Vacuuming of indexes can be very slow.
> 
> > .
> > My plan is as follows.
> > 
> > Add a new option to force index recreation in vacuum
> > and if index recreation is specified.
> 
> Couldn't we auto-recreate indexes based on the number of tuples moved by
> vacuum,

Yes,we could probably do it. But I'm not sure the availability of new
vacuum.

New vacuum would give us a big advantage that
1) Much faster than current if vacuum remove/moves many tuples.
2) Does shrink index files

But in case of abort/crash
1) couldn't choose index scan for the table
2) unique constraints of the table would be lost

I don't know how people estimate this disadvantage.
  
> 
> > Now I'm inclined to use relhasindex of pg_class to
> > validate/invalidate indexes of a table at once.
> 
> There are a few calls to CatalogIndexInsert() that know the 
> system table they
> are using and know it has indexes, so it does not check that field.  You
> could add cases for that.
>

I think there aren't so many places to check.
I would examine it if my idea is OK.

Regards.

Hiroshi Inoue
Inoue@tpf.co.jp

************

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From: Bruce Momjian <pgman@candle.pha.pa.us>
Message-Id: <200001190150.UAA11421@candle.pha.pa.us>
Subject: Re: [HACKERS] Index recreation in vacuum
In-Reply-To: <000201bf621a$6b9baf20$2801007e@tpf.co.jp> from Hiroshi Inoue at
	"Jan 19, 2000 10:13:40 am"
To: Hiroshi Inoue <Inoue@tpf.co.jp>
Date: Tue, 18 Jan 2000 20:50:50 -0500 (EST)
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> > > Add a new option to force index recreation in vacuum
> > > and if index recreation is specified.
> > 
> > Couldn't we auto-recreate indexes based on the number of tuples moved by
> > vacuum,
> 
> Yes,we could probably do it. But I'm not sure the availability of new
> vacuum.
> 
> New vacuum would give us a big advantage that
> 1) Much faster than current if vacuum remove/moves many tuples.
> 2) Does shrink index files
> 
> But in case of abort/crash
> 1) couldn't choose index scan for the table
> 2) unique constraints of the table would be lost
> 
> I don't know how people estimate this disadvantage.

That's why I was recommending rename().  The actual window of
vunerability goes from perhaps hours to fractions of a second.

In fact, if I understand this right, you could make the vulerability
zero by just performing the rename as one operation.

In fact, for REINDEX cases where you don't have a lock on the entire
table as you do in vacuum, you could reindex the table with a simple
read-lock on the base table and index, and move the new index into place
with the users seeing no change.  Only people traversing the index
during the change would have a problem.  You just need an exclusive
access on the index for the duration of the rename() so no one is
traversing the index during the rename().

Destroying the index and recreating opens a large time span that there
is no index, and you have to jury-rig something so people don't try to
use the index.  With rename() you just put the new index in place with
one operation.  Just don't let people traverse the index during the
change.  The pointers to the heap tuples is the same in both indexes.

In fact, with WAL, we will allow multiple physical files for the same
table by appending the table oid to the file name.  In this case, the
old index could be deleted by rename, and people would continue to use
the old index until they deleted the open file pointers.  Not sure how
this works in practice because new tuples would not be inserted into the
old copy of the index.


-- 
  Bruce Momjian                        |  http://www.op.net/~candle
  pgman@candle.pha.pa.us               |  (610) 853-3000
  +  If your life is a hard drive,     |  830 Blythe Avenue
  +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026

************

From pgman Tue Jan 18 20:04:11 2000
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From: Bruce Momjian <pgman>
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Subject: Re: [HACKERS] Index recreation in vacuum
In-Reply-To: <200001190150.UAA11421@candle.pha.pa.us> from Bruce Momjian at "Jan
	18, 2000 08:50:50 pm"
To: Bruce Momjian <pgman@candle.pha.pa.us>
Date: Tue, 18 Jan 2000 21:04:11 -0500 (EST)
CC: Hiroshi Inoue <Inoue@tpf.co.jp>,
        pgsql-hackers <pgsql-hackers@postgreSQL.org>
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> > I don't know how people estimate this disadvantage.
> 
> That's why I was recommending rename().  The actual window of
> vunerability goes from perhaps hours to fractions of a second.
> 
> In fact, if I understand this right, you could make the vulerability
> zero by just performing the rename as one operation.
> 
> In fact, for REINDEX cases where you don't have a lock on the entire
> table as you do in vacuum, you could reindex the table with a simple
> read-lock on the base table and index, and move the new index into place
> with the users seeing no change.  Only people traversing the index
> during the change would have a problem.  You just need an exclusive
> access on the index for the duration of the rename() so no one is
> traversing the index during the rename().
> 
> Destroying the index and recreating opens a large time span that there
> is no index, and you have to jury-rig something so people don't try to
> use the index.  With rename() you just put the new index in place with
> one operation.  Just don't let people traverse the index during the
> change.  The pointers to the heap tuples is the same in both indexes.
> 
> In fact, with WAL, we will allow multiple physical files for the same
> table by appending the table oid to the file name.  In this case, the
> old index could be deleted by rename, and people would continue to use
> the old index until they deleted the open file pointers.  Not sure how
> this works in practice because new tuples would not be inserted into the
> old copy of the index.

Maybe I am all wrong here.  Maybe most of the advantage of rename() are
meaningless with reindex using during vacuum, which is the most
important use of reindex.

Let's look at index using during vacuum.  Right now, how does vacuum
handle indexes when it moves a tuple?  Does it do each index update as
it moves a tuple?  Is that why it is so slow?

If we don't do that and vacuum fails, what state is the table left in? 
If we don't update the index for every tuple, the index is invalid in a
vacuum failure.  rename() is not going to help us here.  It keeps the
old index around, but the index is invalid anyway, right?


-- 
  Bruce Momjian                        |  http://www.op.net/~candle
  pgman@candle.pha.pa.us               |  (610) 853-3000
  +  If your life is a hard drive,     |  830 Blythe Avenue
  +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026

From Inoue@tpf.co.jp Tue Jan 18 20:18:48 2000
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From: "Hiroshi Inoue" <Inoue@tpf.co.jp>
To: "Bruce Momjian" <pgman@candle.pha.pa.us>
Cc: "pgsql-hackers" <pgsql-hackers@postgreSQL.org>
Subject: RE: [HACKERS] Index recreation in vacuum
Date: Wed, 19 Jan 2000 11:23:55 +0900
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> -----Original Message-----
> From: Bruce Momjian [mailto:pgman@candle.pha.pa.us]
>
> > > I don't know how people estimate this disadvantage.
> >
> > That's why I was recommending rename().  The actual window of
> > vunerability goes from perhaps hours to fractions of a second.
> >
> > In fact, if I understand this right, you could make the vulerability
> > zero by just performing the rename as one operation.
> >
> > In fact, for REINDEX cases where you don't have a lock on the entire
> > table as you do in vacuum, you could reindex the table with a simple
> > read-lock on the base table and index, and move the new index into place
> > with the users seeing no change.  Only people traversing the index
> > during the change would have a problem.  You just need an exclusive
> > access on the index for the duration of the rename() so no one is
> > traversing the index during the rename().
> >
> > Destroying the index and recreating opens a large time span that there
> > is no index, and you have to jury-rig something so people don't try to
> > use the index.  With rename() you just put the new index in place with
> > one operation.  Just don't let people traverse the index during the
> > change.  The pointers to the heap tuples is the same in both indexes.
> >
> > In fact, with WAL, we will allow multiple physical files for the same
> > table by appending the table oid to the file name.  In this case, the
> > old index could be deleted by rename, and people would continue to use
> > the old index until they deleted the open file pointers.  Not sure how
> > this works in practice because new tuples would not be inserted into the
> > old copy of the index.
>
> Maybe I am all wrong here.  Maybe most of the advantage of rename() are
> meaningless with reindex using during vacuum, which is the most
> important use of reindex.
>
> Let's look at index using during vacuum.  Right now, how does vacuum
> handle indexes when it moves a tuple?  Does it do each index update as
> it moves a tuple?  Is that why it is so slow?
>

Yes,I believe so.  It's necessary to keep consistency between heap
table and indexes even in case of abort/crash.
As far as I see,it has been a big charge for vacuum.

Regards.

Hiroshi Inoue
Inoue@tpf.co.jp


From owner-pgsql-hackers@hub.org Tue Jan 18 20:53:49 2000
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From: Bruce Momjian <pgman@candle.pha.pa.us>
Message-Id: <200001190245.VAA13040@candle.pha.pa.us>
Subject: Re: [HACKERS] Index recreation in vacuum
In-Reply-To: <000801bf6224$3bfdd9a0$2801007e@tpf.co.jp> from Hiroshi Inoue at
	"Jan 19, 2000 11:23:55 am"
To: Hiroshi Inoue <Inoue@tpf.co.jp>
Date: Tue, 18 Jan 2000 21:45:27 -0500 (EST)
CC: pgsql-hackers <pgsql-hackers@postgreSQL.org>
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> > > In fact, for REINDEX cases where you don't have a lock on the entire
> > > table as you do in vacuum, you could reindex the table with a simple
> > > read-lock on the base table and index, and move the new index into place
> > > with the users seeing no change.  Only people traversing the index
> > > during the change would have a problem.  You just need an exclusive
> > > access on the index for the duration of the rename() so no one is
> > > traversing the index during the rename().
> > >
> > > Destroying the index and recreating opens a large time span that there
> > > is no index, and you have to jury-rig something so people don't try to
> > > use the index.  With rename() you just put the new index in place with
> > > one operation.  Just don't let people traverse the index during the
> > > change.  The pointers to the heap tuples is the same in both indexes.
> > >
> > > In fact, with WAL, we will allow multiple physical files for the same
> > > table by appending the table oid to the file name.  In this case, the
> > > old index could be deleted by rename, and people would continue to use
> > > the old index until they deleted the open file pointers.  Not sure how
> > > this works in practice because new tuples would not be inserted into the
> > > old copy of the index.
> >
> > Maybe I am all wrong here.  Maybe most of the advantage of rename() are
> > meaningless with reindex using during vacuum, which is the most
> > important use of reindex.
> >
> > Let's look at index using during vacuum.  Right now, how does vacuum
> > handle indexes when it moves a tuple?  Does it do each index update as
> > it moves a tuple?  Is that why it is so slow?
> >
> 
> Yes,I believe so.  It's necessary to keep consistency between heap
> table and indexes even in case of abort/crash.
> As far as I see,it has been a big charge for vacuum.

OK, how about making a copy of the heap table before starting vacuum,
moving all the tuples in that copy, create new index, and then move the
new heap and indexes over the old version.  We already have an exclusive
lock on the table.  That would be 100% reliable, with the disadvantage
of using 2x the disk space.  Seems like a big win.

-- 
  Bruce Momjian                        |  http://www.op.net/~candle
  pgman@candle.pha.pa.us               |  (610) 853-3000
  +  If your life is a hard drive,     |  830 Blythe Avenue
  +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026

************

From owner-pgsql-hackers@hub.org Tue Jan 18 21:15:24 2000
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From: Bruce Momjian <pgman@candle.pha.pa.us>
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Subject: Re: [HACKERS] Index recreation in vacuum
In-Reply-To: <000f01bf622a$bf423940$2801007e@tpf.co.jp> from Hiroshi Inoue at
	"Jan 19, 2000 12:10:32 pm"
To: Hiroshi Inoue <Inoue@tpf.co.jp>
Date: Tue, 18 Jan 2000 22:08:25 -0500 (EST)
CC: pgsql-hackers <pgsql-hackers@postgreSQL.org>
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> I heard from someone that old vacuum had been like so.
> Probably 2x disk space for big tables was a big disadvantage.

That's interesting.

> 
> In addition,rename(),unlink(),mv aren't preferable for transaction
> control as far as I see. We couldn't avoid inconsistency using
> those OS functions.

I disagree.  Vacuum can't be rolled back anyway in the sense you can
bring back expire tuples, though I have no idea why you would want to.

You have an exclusive lock on the table.  Putting new heap/indexes in
place that match and have no expired tuples seems like it can not fail
in any situation.

Of course, the buffers of the old table have to be marked as invalid,
but with an exclusive lock, that is not a problem.  I am sure we do that
anyway�in vacuum.

> We have to wait the change of relation file naming if copying
> vacuum is needed.
> Under the spec we need not rename(),mv etc.

Sorry, I don't agree, yet...

-- 
  Bruce Momjian                        |  http://www.op.net/~candle
  pgman@candle.pha.pa.us               |  (610) 853-3000
  +  If your life is a hard drive,     |  830 Blythe Avenue
  +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026

************

From Inoue@tpf.co.jp Tue Jan 18 21:05:23 2000
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From: "Hiroshi Inoue" <Inoue@tpf.co.jp>
To: "Bruce Momjian" <pgman@candle.pha.pa.us>
Cc: "pgsql-hackers" <pgsql-hackers@postgreSQL.org>
Subject: RE: [HACKERS] Index recreation in vacuum
Date: Wed, 19 Jan 2000 12:10:32 +0900
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> -----Original Message-----
> From: Bruce Momjian [mailto:pgman@candle.pha.pa.us]
> > >
> > > Maybe I am all wrong here.  Maybe most of the advantage of
> rename() are
> > > meaningless with reindex using during vacuum, which is the most
> > > important use of reindex.
> > >
> > > Let's look at index using during vacuum.  Right now, how does vacuum
> > > handle indexes when it moves a tuple?  Does it do each index update as
> > > it moves a tuple?  Is that why it is so slow?
> > >
> >
> > Yes,I believe so.  It's necessary to keep consistency between heap
> > table and indexes even in case of abort/crash.
> > As far as I see,it has been a big charge for vacuum.
>
> OK, how about making a copy of the heap table before starting vacuum,
> moving all the tuples in that copy, create new index, and then move the
> new heap and indexes over the old version.  We already have an exclusive
> lock on the table.  That would be 100% reliable, with the disadvantage
> of using 2x the disk space.  Seems like a big win.
>

I heard from someone that old vacuum had been like so.
Probably 2x disk space for big tables was a big disadvantage.

In addition,rename(),unlink(),mv aren't preferable for transaction
control as far as I see. We couldn't avoid inconsistency using
those OS functions.
We have to wait the change of relation file naming if copying
vacuum is needed.
Under the spec we need not rename(),mv etc.

Regards.

Hiroshi Inoue
Inoue@tpf.co.jp




From dms@wplus.net Wed Jan 19 15:30:40 2000
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From: Dmitry Samersoff <dms@wplus.net>
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Subject: Re: [HACKERS] Index recreation in vacuum
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Hiroshi Inoue wrote:
> > > Yes,I believe so.  It's necessary to keep consistency between heap
> > > table and indexes even in case of abort/crash.
> > > As far as I see,it has been a big charge for vacuum.
> >
> > OK, how about making a copy of the heap table before starting vacuum,
> > moving all the tuples in that copy, create new index, and then move the
> > new heap and indexes over the old version.  We already have an exclusive
> > lock on the table.  That would be 100% reliable, with the disadvantage
> > of using 2x the disk space.  Seems like a big win.
> >
> 
> I heard from someone that old vacuum had been like so.
> Probably 2x disk space for big tables was a big disadvantage.

Yes, It is critical.

How about sequence like this:

* Drop indices (keeping somewhere index descriptions)
* vacuuming table
* recreate indices

If something crash, user have been noticed 
to re-run vacuum or recreate indices by hand 
when system restarts.

I use script like described above for vacuuming
 - it really increase vacuum performance for large table.


-- 
Dmitry Samersoff, DM\S
dms@wplus.net http://devnull.wplus.net
* there will come soft rains

From dms@wplus.net Wed Jan 19 15:42:49 2000
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Bruce Momjian wrote:
> 
> We need two things:
> 

>         auto-create index on startup

IMHO, It have to be controlled by user, because creating large index 
can take a number of hours. Sometimes it's better to live without
indices
at all, and then build it by hand after workday end.


-- 
Dmitry Samersoff, DM\S
dms@wplus.net http://devnull.wplus.net
* there will come soft rains

From owner-pgsql-hackers@hub.org Thu Jan 20 23:51:34 2000
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From: Bruce Momjian <pgman@candle.pha.pa.us>
Message-Id: <200001210543.AAA13592@candle.pha.pa.us>
Subject: [HACKERS] vacuum timings
To: Tom Lane <tgl@sss.pgh.pa.us>
Date: Fri, 21 Jan 2000 00:43:49 -0500 (EST)
CC: PostgreSQL-development <pgsql-hackers@postgreSQL.org>
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I loaded 10,000,000 rows into CREATE TABLE test (x INTEGER);  Table is
400MB and index is 160MB.

With index on the single in4 column, I got:
	 78 seconds for a vacuum
	121 seconds for vacuum after deleting a single row
	662 seconds for vacuum after deleting the entire table

With no index, I got:
	 43 seconds for a vacuum
	 43 seconds for vacuum after deleting a single row
	 43 seconds for vacuum after deleting the entire table

I find this quite interesting.

-- 
  Bruce Momjian                        |  http://www.op.net/~candle
  pgman@candle.pha.pa.us               |  (610) 853-3000
  +  If your life is a hard drive,     |  830 Blythe Avenue
  +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026

************

From owner-pgsql-hackers@hub.org Fri Jan 21 00:34:56 2000
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From: Vadim Mikheev <vadim@krs.ru>
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CC: Tom Lane <tgl@sss.pgh.pa.us>,
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Subject: Re: [HACKERS] vacuum timings
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Bruce Momjian wrote:
> 
> I loaded 10,000,000 rows into CREATE TABLE test (x INTEGER);  Table is
> 400MB and index is 160MB.
> 
> With index on the single in4 column, I got:
>          78 seconds for a vacuum
>         121 seconds for vacuum after deleting a single row
>         662 seconds for vacuum after deleting the entire table
> 
> With no index, I got:
>          43 seconds for a vacuum
>          43 seconds for vacuum after deleting a single row
>          43 seconds for vacuum after deleting the entire table

Wi/wo -F ?

Vadim

************

From vadim@krs.ru Fri Jan 21 00:26:33 2000
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CC: Tom Lane <tgl@sss.pgh.pa.us>,
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Subject: Re: [HACKERS] vacuum timings
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Bruce Momjian wrote:
> 
> I loaded 10,000,000 rows into CREATE TABLE test (x INTEGER);  Table is
> 400MB and index is 160MB.
> 
> With index on the single in4 column, I got:
>          78 seconds for a vacuum
>         121 seconds for vacuum after deleting a single row
>         662 seconds for vacuum after deleting the entire table
> 
> With no index, I got:
>          43 seconds for a vacuum
>          43 seconds for vacuum after deleting a single row
>          43 seconds for vacuum after deleting the entire table

Wi/wo -F ?

Vadim

From Inoue@tpf.co.jp Fri Jan 21 00:40:35 2000
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From: "Hiroshi Inoue" <Inoue@tpf.co.jp>
To: "Bruce Momjian" <pgman@candle.pha.pa.us>
Cc: "PostgreSQL-development" <pgsql-hackers@postgreSQL.org>,
        "Tom Lane" <tgl@sss.pgh.pa.us>
Subject: RE: [HACKERS] vacuum timings
Date: Fri, 21 Jan 2000 15:46:15 +0900
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> -----Original Message-----
> From: owner-pgsql-hackers@postgreSQL.org
> [mailto:owner-pgsql-hackers@postgreSQL.org]On Behalf Of Bruce Momjian
> 
> I loaded 10,000,000 rows into CREATE TABLE test (x INTEGER);  Table is
> 400MB and index is 160MB.
> 
> With index on the single in4 column, I got:
> 	 78 seconds for a vacuum
		vc_vaconeind() is called once

> 	121 seconds for vacuum after deleting a single row
		vc_vaconeind() is called twice

Hmmm,vc_vaconeind() takes pretty long time even if it does little. 

> 	662 seconds for vacuum after deleting the entire table
>

How about half of the rows deleted case ?
It would take longer time.

Regards.

Hiroshi Inoue
Inoue@tpf.co.jp

From owner-pgsql-hackers@hub.org Fri Jan 21 12:00:49 2000
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From: Bruce Momjian <pgman@candle.pha.pa.us>
Message-Id: <200001211751.MAA12106@candle.pha.pa.us>
Subject: [HACKERS] Re: vacuum timings
In-Reply-To: <3641.948433911@sss.pgh.pa.us> from Tom Lane at "Jan 21, 2000 00:51:51
	am"
To: Tom Lane <tgl@sss.pgh.pa.us>
Date: Fri, 21 Jan 2000 12:51:53 -0500 (EST)
CC: PostgreSQL-development <pgsql-hackers@postgreSQL.org>
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> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > I loaded 10,000,000 rows into CREATE TABLE test (x INTEGER);  Table is
> > 400MB and index is 160MB.
> 
> > With index on the single in4 column, I got:
> > 	 78 seconds for a vacuum
> > 	121 seconds for vacuum after deleting a single row
> > 	662 seconds for vacuum after deleting the entire table
> 
> > With no index, I got:
> > 	 43 seconds for a vacuum
> > 	 43 seconds for vacuum after deleting a single row
> > 	 43 seconds for vacuum after deleting the entire table
> 
> > I find this quite interesting.
> 
> How long does it take to create the index on your setup --- ie,
> if vacuum did a drop/create index, would it be competitive?

OK, new timings with -F enabled:

	index	no index
	519	same	load	
	247	"	first vacuum
	40	"	other vacuums
	
	1222	X	index creation
	90	X	first vacuum
	80	X	other vacuums
	
	<1	90	delete one row
	121	38	vacuum after delete 1 row
	
	346	344	delete all rows
	440	44	first vacuum
	20	<1	other vacuums(index is still same size)

Conclusions:

	o  indexes never get smaller
	o  drop/recreate index is slower than vacuum of indexes

What other conclusions can be made?

-- 
  Bruce Momjian                        |  http://www.op.net/~candle
  pgman@candle.pha.pa.us               |  (610) 853-3000
  +  If your life is a hard drive,     |  830 Blythe Avenue
  +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026

************

From scrappy@hub.org Fri Jan 21 12:45:38 2000
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From: The Hermit Hacker <scrappy@hub.org>
To: Bruce Momjian <pgman@candle.pha.pa.us>
cc: Tom Lane <tgl@sss.pgh.pa.us>,
        PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] Re: vacuum timings
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On Fri, 21 Jan 2000, Bruce Momjian wrote:

> OK, new timings with -F enabled:
> 
> 	index	no index
> 	519	same	load	
> 	247	"	first vacuum
> 	40	"	other vacuums
> 	
> 	1222	X	index creation
> 	90	X	first vacuum
> 	80	X	other vacuums
> 	
> 	<1	90	delete one row
> 	121	38	vacuum after delete 1 row
> 	
> 	346	344	delete all rows
> 	440	44	first vacuum
> 	20	<1	other vacuums(index is still same size)
> 
> Conclusions:
> 
> 	o  indexes never get smaller

this one, I thought, was a known?  if I remember right, Vadim changed it
so that space was reused, but index never shrunk in size ... no?

Marc G. Fournier                   ICQ#7615664               IRC Nick: Scrappy
Systems Administrator @ hub.org 
primary: scrappy@hub.org           secondary: scrappy@{freebsd|postgresql}.org 


From tgl@sss.pgh.pa.us Fri Jan 21 13:06:35 2000
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Subject: Re: vacuum timings 
In-reply-to: <200001211751.MAA12106@candle.pha.pa.us> 
References: <200001211751.MAA12106@candle.pha.pa.us>
Comments: In-reply-to Bruce Momjian <pgman@candle.pha.pa.us>
	message dated "Fri, 21 Jan 2000 12:51:53 -0500"
Date: Fri, 21 Jan 2000 14:06:31 -0500
Message-ID: <16498.948481591@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
Status: RO

Bruce Momjian <pgman@candle.pha.pa.us> writes:
> Conclusions:
> 	o  indexes never get smaller

Which we knew...

> 	o  drop/recreate index is slower than vacuum of indexes

Quite a few people have reported finding the opposite in practice.
You should probably try vacuuming after deleting or updating some
fraction of the rows, rather than just the all or none cases.

			regards, tom lane

From dms@wplus.net Fri Jan 21 13:51:27 2000
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Date: Fri, 21 Jan 2000 22:48:50 +0300
From: Dmitry Samersoff <dms@wplus.net>
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CC: Bruce Momjian <pgman@candle.pha.pa.us>,
        PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] Re: vacuum timings
References: <200001211751.MAA12106@candle.pha.pa.us> <16498.948481591@sss.pgh.pa.us>
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Tom Lane wrote:
> 
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > Conclusions:
> >       o  indexes never get smaller
> 
> Which we knew...
> 
> >       o  drop/recreate index is slower than vacuum of indexes
> 
> Quite a few people have reported finding the opposite in practice.

I'm one of them. On 1,5 GB table with three indices it about twice
slowly.
Probably becouse vacuuming indices brakes system cache policy.
(FreeBSD 3.3)



-- 
Dmitry Samersoff, DM\S
dms@wplus.net http://devnull.wplus.net
* there will come soft rains

From owner-pgsql-hackers@hub.org Fri Jan 21 14:04:08 2000
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From: Bruce Momjian <pgman@candle.pha.pa.us>
Message-Id: <200001211954.OAA15772@candle.pha.pa.us>
Subject: Re: [HACKERS] Re: vacuum timings
In-Reply-To: <3888B822.28F79A1F@wplus.net> from Dmitry Samersoff at "Jan 21,
	2000 10:48:50 pm"
To: Dmitry Samersoff <dms@wplus.net>
Date: Fri, 21 Jan 2000 14:54:21 -0500 (EST)
CC: Tom Lane <tgl@sss.pgh.pa.us>,
        PostgreSQL-development <pgsql-hackers@postgreSQL.org>
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[Charset koi8-r unsupported, filtering to ASCII...]
> Tom Lane wrote:
> > 
> > Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > > Conclusions:
> > >       o  indexes never get smaller
> > 
> > Which we knew...
> > 
> > >       o  drop/recreate index is slower than vacuum of indexes
> > 
> > Quite a few people have reported finding the opposite in practice.
> 
> I'm one of them. On 1,5 GB table with three indices it about twice
> slowly.
> Probably becouse vacuuming indices brakes system cache policy.
> (FreeBSD 3.3)

OK, we are researching what things can be done to improve this.  We are
toying with:

	lock table for less duration, or read lock
	creating another copy of heap/indexes, and rename() over old files
	improving heap vacuum speed
	improving index vacuum speed
	moving analyze out of vacuum
	

-- 
  Bruce Momjian                        |  http://www.op.net/~candle
  pgman@candle.pha.pa.us               |  (610) 853-3000
  +  If your life is a hard drive,     |  830 Blythe Avenue
  +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026

************

From scrappy@hub.org Fri Jan 21 14:12:16 2000
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Date: Fri, 21 Jan 2000 16:12:25 -0400 (AST)
From: The Hermit Hacker <scrappy@hub.org>
To: Bruce Momjian <pgman@candle.pha.pa.us>
cc: Dmitry Samersoff <dms@wplus.net>, Tom Lane <tgl@sss.pgh.pa.us>,
        PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] Re: vacuum timings
In-Reply-To: <200001211954.OAA15772@candle.pha.pa.us>
Message-ID: <Pine.BSF.4.21.0001211607080.23487-100000@thelab.hub.org>
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Status: RO

On Fri, 21 Jan 2000, Bruce Momjian wrote:

> [Charset koi8-r unsupported, filtering to ASCII...]
> > Tom Lane wrote:
> > > 
> > > Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > > > Conclusions:
> > > >       o  indexes never get smaller
> > > 
> > > Which we knew...
> > > 
> > > >       o  drop/recreate index is slower than vacuum of indexes
> > > 
> > > Quite a few people have reported finding the opposite in practice.
> > 
> > I'm one of them. On 1,5 GB table with three indices it about twice
> > slowly.
> > Probably becouse vacuuming indices brakes system cache policy.
> > (FreeBSD 3.3)
> 
> OK, we are researching what things can be done to improve this.  We are
> toying with:
> 
> 	lock table for less duration, or read lock

if there is some way that we can work around the bug that I believe Tom
found with removing the lock altogether (ie. makig use of MVCC), I think
that would be the best option ... if not possible, at least get things
down to a table lock vs the whole database?

a good example is the udmsearch that we are using on the site ... it uses
multiple tables to store the dictionary, each representing words of X size
... if I'm searching on a 4 letter word, and the whole database is locked
while it is working on the dictionary with 8 letter words, I'm sitting
there idle ... at least if we only locked the 8 letter table, everyone not
doing 8 letter searches can go on their merry way ...

Slightly longer vacuum's, IMHO, are acceptable if, to the end users, its
as transparent as possible ... locking per table would be slightly slower,
I think, because once a table is finished, the next table would need to
have an exclusive lock put on it before starting, so you'd have to
possibly wait for that...?

> 	creating another copy of heap/indexes, and rename() over old files

sounds to me like introducing a large potential for error here ...

> 	moving analyze out of vacuum

I think that should be done anyway ... if we ever get to the point that
we're able to re-use rows in tables, then that would eliminate the
immediate requirement for vacuum, but still retain a requirement for a
periodic analyze ... no?

Marc G. Fournier                   ICQ#7615664               IRC Nick: Scrappy
Systems Administrator @ hub.org 
primary: scrappy@hub.org           secondary: scrappy@{freebsd|postgresql}.org 


From tgl@sss.pgh.pa.us Fri Jan 21 16:02:07 2000
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To: The Hermit Hacker <scrappy@hub.org>
cc: Bruce Momjian <pgman@candle.pha.pa.us>,
        PostgreSQL-development <pgsql-hackers@postgreSQL.org>
Subject: Re: [HACKERS] Re: vacuum timings 
In-reply-to: <Pine.BSF.4.21.0001211607080.23487-100000@thelab.hub.org> 
References: <Pine.BSF.4.21.0001211607080.23487-100000@thelab.hub.org>
Comments: In-reply-to The Hermit Hacker <scrappy@hub.org>
	message dated "Fri, 21 Jan 2000 16:12:25 -0400"
Date: Fri, 21 Jan 2000 17:02:06 -0500
Message-ID: <9694.948492126@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
Status: RO

The Hermit Hacker <scrappy@hub.org> writes:
>> lock table for less duration, or read lock

> if there is some way that we can work around the bug that I believe Tom
> found with removing the lock altogether (ie. makig use of MVCC), I think
> that would be the best option ... if not possible, at least get things
> down to a table lock vs the whole database?

Huh?  VACUUM only requires an exclusive lock on the table it is
currently vacuuming; there's no database-wide lock.

Even a single-table exclusive lock is bad, of course, if it's a large
table that's critical to a 24x7 application.  Bruce was talking about
the possibility of having VACUUM get just a write lock on the table;
other backends could still read it, but not write it, during the vacuum
process.  That'd be a considerable step forward for 24x7 applications,
I think.

It looks like that could be done if we rewrote the table as a new file
(instead of compacting-in-place), but there's a problem when it comes
time to rename the new files into place.  At that point you'd need to
get an exclusive lock to ensure all the readers are out of the table too
--- and upgrading from a plain lock to an exclusive lock is a well-known
recipe for deadlocks.  Not sure if this can be solved.

			regards, tom lane

From tgl@sss.pgh.pa.us Fri Jan 21 22:50:34 2000
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To: Bruce Momjian <pgman@candle.pha.pa.us>
cc: PostgreSQL-development <pgsql-hackers@postgreSQL.org>
Subject: Re: vacuum timings 
In-reply-to: <200001211751.MAA12106@candle.pha.pa.us> 
References: <200001211751.MAA12106@candle.pha.pa.us>
Comments: In-reply-to Bruce Momjian <pgman@candle.pha.pa.us>
	message dated "Fri, 21 Jan 2000 12:51:53 -0500"
Date: Fri, 21 Jan 2000 23:50:13 -0500
Message-ID: <19678.948516613@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
Status: ROr

Bruce Momjian <pgman@candle.pha.pa.us> writes:
> Conclusions:
> 	o  drop/recreate index is slower than vacuum of indexes

BTW, I did some profiling of CREATE INDEX this evening (quite
unintentionally actually; I was interested in COPY IN, but the pg_dump
script I used as driver happened to create some indexes too).  I was
startled to discover that 60% of the runtime of CREATE INDEX is spent in
_bt_invokestrat (which is called from tuplesort.c's comparetup_index,
and exists only to figure out which specific comparison routine to call).
Of this, a whopping 4% was spent in the useful subroutine, int4gt.  All
the rest went into lookup and validation checks that by rights should be
done once per index creation, not once per comparison.

In short: a fairly straightforward bit of optimization will eliminate
circa 50% of the CPU time consumed by CREATE INDEX.  All we need is to
figure out where to cache the lookup results.  The optimization would
improve insertions and lookups in indexes, as well, if we can cache
the lookup results in those scenarios.

This was for a table small enough that tuplesort.c could do the sort
entirely in memory, so I'm sure the gains would be smaller for a large
table that requires a disk-based sort.  Still, it seems worth looking
into...

			regards, tom lane

From owner-pgsql-hackers@hub.org Sat Jan 22 02:31:03 2000
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From: "Hiroshi Inoue" <Inoue@tpf.co.jp>
To: "Tom Lane" <tgl@sss.pgh.pa.us>, "Bruce Momjian" <pgman@candle.pha.pa.us>
Cc: "PostgreSQL-development" <pgsql-hackers@postgresql.org>
Subject: RE: [HACKERS] Re: vacuum timings 
Date: Sat, 22 Jan 2000 17:15:37 +0900
Message-ID: <NDBBIJLOILGIKBGDINDFIEEACCAA.Inoue@tpf.co.jp>
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> -----Original Message-----
> From: owner-pgsql-hackers@postgresql.org
> [mailto:owner-pgsql-hackers@postgresql.org]On Behalf Of Tom Lane
> 
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > Conclusions:
> > 	o  indexes never get smaller
> 
> Which we knew...
> 
> > 	o  drop/recreate index is slower than vacuum of indexes
> 
> Quite a few people have reported finding the opposite in practice.
> You should probably try vacuuming after deleting or updating some
> fraction of the rows, rather than just the all or none cases.
>

Vacuum after delelting all rows isn't a worst case.
There's no moving in that case and vacuum doesn't need to call
index_insert() corresponding to the moving of heap tuples.

Vacuum after deleting half of rows may be one of the worst case.
In this case,index_delete() is called as many times as 'delete all'
case and expensive index_insert() is called for moved_in tuples.

Regards.

Hiroshi Inoue
Inoue@tpf.co.jp 

************

From tgl@sss.pgh.pa.us Sat Jan 22 10:31:02 2000
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To: "Hiroshi Inoue" <Inoue@tpf.co.jp>
cc: "Bruce Momjian" <pgman@candle.pha.pa.us>,
        "PostgreSQL-development" <pgsql-hackers@postgreSQL.org>
Subject: Re: [HACKERS] Re: vacuum timings 
In-reply-to: <NDBBIJLOILGIKBGDINDFIEEACCAA.Inoue@tpf.co.jp> 
References: <NDBBIJLOILGIKBGDINDFIEEACCAA.Inoue@tpf.co.jp>
Comments: In-reply-to "Hiroshi Inoue" <Inoue@tpf.co.jp>
	message dated "Sat, 22 Jan 2000 17:15:37 +0900"
Date: Sat, 22 Jan 2000 11:11:25 -0500
Message-ID: <20566.948557485@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
Status: RO

"Hiroshi Inoue" <Inoue@tpf.co.jp> writes:
> Vacuum after deleting half of rows may be one of the worst case.

Or equivalently, vacuum after updating all the rows.

			regards, tom lane

From tgl@sss.pgh.pa.us Thu Jan 20 23:51:49 2000
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To: Bruce Momjian <pgman@candle.pha.pa.us>
cc: PostgreSQL-development <pgsql-hackers@postgreSQL.org>
Subject: Re: vacuum timings 
In-reply-to: <200001210543.AAA13592@candle.pha.pa.us> 
References: <200001210543.AAA13592@candle.pha.pa.us>
Comments: In-reply-to Bruce Momjian <pgman@candle.pha.pa.us>
	message dated "Fri, 21 Jan 2000 00:43:49 -0500"
Date: Fri, 21 Jan 2000 00:51:51 -0500
Message-ID: <3641.948433911@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
Status: ROr

Bruce Momjian <pgman@candle.pha.pa.us> writes:
> I loaded 10,000,000 rows into CREATE TABLE test (x INTEGER);  Table is
> 400MB and index is 160MB.

> With index on the single in4 column, I got:
> 	 78 seconds for a vacuum
> 	121 seconds for vacuum after deleting a single row
> 	662 seconds for vacuum after deleting the entire table

> With no index, I got:
> 	 43 seconds for a vacuum
> 	 43 seconds for vacuum after deleting a single row
> 	 43 seconds for vacuum after deleting the entire table

> I find this quite interesting.

How long does it take to create the index on your setup --- ie,
if vacuum did a drop/create index, would it be competitive?

			regards, tom lane

From pgsql-hackers-owner+M5909@hub.org Thu Aug 17 20:15:33 2000
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Date: Thu, 17 Aug 2000 17:01:18 -0700
From: Alfred Perlstein <bright@wintelcom.net>
To: pgsql-hackers@postgresql.org
Subject: [HACKERS] VACUUM optimization ideas.
Message-ID: <20000817170118.K4854@fw.wintelcom.net>
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Here's two ideas I had for optimizing vacuum, I apologize in advance
if the ideas presented here are niave and don't take into account
the actual code that makes up postgresql.

================

#1

Reducing the time vacuum must hold an exlusive lock on a table:

The idea is that since rows are marked deleted it's ok for the
vacuum to fill them with data from the tail of the table as
long as no transaction is in progress that has started before
the row was deleted.

This may allow the vacuum process to copyback all the data without
a lock, when all the copying is done it then aquires an exlusive lock
and does this:

Aquire an exclusive lock.
Walk all the deleted data marking it as current.
Truncate the table.
Release the lock.

Since the data is still marked invalid (right?) even if valid data
is copied into the space it should be ignored as long as there's no
transaction occurring that started before the data was invalidated.

================

#2

Reducing the amount of scanning a vaccum must do:

It would make sense that if a value of the earliest deleted chunk
was kept in a table then vacuum would not have to scan the entire
table in order to work, it would only need to start at the 'earliest'
invalidated row.

The utility of this (at least for us) is that we have several tables
that will grow to hundreds of megabytes, however changes will only
happen at the tail end (recently added rows).  If we could reduce the
amount of time spent in a vacuum state it would help us a lot.

================

I'm wondering if these ideas make sense and may help at all.

thanks,
--
-Alfred Perlstein - [bright@wintelcom.net|alfred@freebsd.org]

From pgsql-hackers-owner+M5912@hub.org Fri Aug 18 01:36:14 2000
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To: Alfred Perlstein <bright@wintelcom.net>, pgsql-hackers@postgresql.org
Subject: Re: [HACKERS] VACUUM optimization ideas.
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Alfred Perlstein wrote:
> #1
> 
> Reducing the time vacuum must hold an exlusive lock on a table:
> 
> The idea is that since rows are marked deleted it's ok for the
> vacuum to fill them with data from the tail of the table as
> long as no transaction is in progress that has started before
> the row was deleted.
> 
> This may allow the vacuum process to copyback all the data without
> a lock, when all the copying is done it then aquires an exlusive lock
> and does this:
> 
> Aquire an exclusive lock.
> Walk all the deleted data marking it as current.
> Truncate the table.
> Release the lock.
> 
> Since the data is still marked invalid (right?) even if valid data
> is copied into the space it should be ignored as long as there's no
> transaction occurring that started before the data was invalidated.

Yes, but nothing prevents newer transactions from modifying the _origin_ side of
the copied data _after_ it was copied, but before the Lock-Walk-Truncate-Unlock
cycle takes place, and so it seems unsafe. Maybe locking each record before
copying it up ...

Regards,
Haroldo.

-- 
----------------------+------------------------
 Haroldo Stenger      | hstenger@ieee.org
 Montevideo, Uruguay. | hstenger@adinet.com.uy
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 Visit UYLUG Web Site: http://www.linux.org.uy
-----------------------------------------------

From pgsql-hackers-owner+M5917@hub.org Fri Aug 18 09:41:33 2000
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Alfred Perlstein wrote:

> The idea is that since rows are marked deleted it's ok for the
> vacuum to fill them with data from the tail of the table as
> long as no transaction is in progress that has started before
> the row was deleted.

Well, isn't one of the advantages of vacuuming in the reordering it
does? With a "fill deleted chunks" logic, we'd have far less order in
the databases. 

> This may allow the vacuum process to copyback all the data without
> a lock, 

Nope. Another process might update the values in between move and mark,
if the record is not locked. We'd either have to write-lock the entire
table for that period, write lock every item as it is moved, or lock,
move and mark on a per-record base. The latter would be slow, but it
could be done in a permanent low priority background process, utilizing
empty CPU cycles. Besides, it probably could not only be done simply
filling from the tail, but also moving up the records in a sorted
fashion. 

> #2
> 
> Reducing the amount of scanning a vaccum must do:
> 
> It would make sense that if a value of the earliest deleted chunk
> was kept in a table then vacuum would not have to scan the entire
> table in order to work, it would only need to start at the 'earliest'
> invalidated row.

Trivial to do. But of course #1 may imply that the physical ordering is
even less likely to be related to the logical ordering in a way where
this helps. 

> The utility of this (at least for us) is that we have several tables
> that will grow to hundreds of megabytes, however changes will only
> happen at the tail end (recently added rows). 

The tail is a relative position - except for the case where you add
temporary records to a constant default set, everything in the tail will
move, at least relatively, to the head after some time.

> If we could reduce the
> amount of time spent in a vacuum state it would help us a lot.

Rather: If we can reduce the time spent in a locked state while
vacuuming, it would help a lot. Being in a vacuum is not the issue -
even permanent vacuuming need not be an issue, if the locks it uses are
suitably  short-time. 

Sevo

-- 
sevo@ip23.net

From pgsql-hackers-owner+M5911@hub.org Thu Aug 17 21:11:20 2000
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To: Chris Bitmead <chrisb@nimrod.itg.telstra.com.au>,
        Ben Adida <ben@openforce.net>
From: Philip Warner <pjw@rhyme.com.au>
Subject: Re: [HACKERS] Inserting a select statement result into another
  table
Cc: Andrew Selle <aselle@upl.cs.wisc.edu>, pgsql-hackers@postgresql.org
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At 09:34 18/08/00 +1000, Chris Bitmead wrote:
>
>He does ask a legitimate question though. If you are going to have a
>LIMIT feature (which of course is not pure SQL), there seems no reason
>you shouldn't be able to insert the result into a table.

This feature is supported by two commercial DBs: Dec/RDB and SQL/Server. I
have no idea if Oracle supports it, but it is such a *useful* feature that
I would be very surprised if it didn't.


>Ben Adida wrote:
>> 
>> What is the purpose you're trying to accomplish with this order by? No
matter what, all the
>> rows where done='f' will be inserted, and you will not be left with any
indication of that
>> order once the rows are in the todolist table.

I don't know what his *purpose* was, but the query should only insert the
first two rows from the select bacause of the limit).

>> Andrew Selle wrote:
>> 
>> > Alright.  My situation is this.  I have a list of things that need to
be done
>> > in a table called tasks.  I have a list of users who will complete
these tasks.
>> > I want these users to be able to come in and "claim" the top 2 most
recent tasks
>> > that have been added.  These tasks then get stored in a table called
todolist
>> > which stores who claimed the task, the taskid, and when the task was
claimed.
>> > For each time someone wants to claim some number of tasks, I want to
do something
>> > like
>> >
>> > INSERT INTO todolist
>> >         SELECT taskid,'1',now()
>> >         FROM tasks
>> >         WHERE done='f'
>> >         ORDER BY submit DESC
>> >         LIMIT 2;

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