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| author | Bruce Momjian <bruce@momjian.us> | 2005-09-02 15:22:44 +0000 |
|---|---|---|
| committer | Bruce Momjian <bruce@momjian.us> | 2005-09-02 15:22:44 +0000 |
| commit | fad7e8edac3348ea14aca379e7b3b2d713746cd3 (patch) | |
| tree | 126bd29aa6d734243a41ac28adb3faa3d9e28f71 /doc/src | |
| parent | 6c0398a73f0a8173996fb0bd638ca6ab7c82e5b6 (diff) | |
| download | postgresql-fad7e8edac3348ea14aca379e7b3b2d713746cd3.tar.gz postgresql-fad7e8edac3348ea14aca379e7b3b2d713746cd3.zip | |
Add text:
* Merge xmin/xmax/cmin/cmax back into three header fields
Before subtransactions, there used to be only three fields needed to
store these four values. This was possible because only the current
transaction looks at the cmin/cmax values. If the current transaction
created and expired the row the fields stored where xmin (same as
xmax), cmin, cmax, and if the transaction was expiring a row from a
another transaction, the fields stored were xmin (cmin was not
needed), xmax, and cmax. Such a system worked because a transaction
could only see committed rows from another transaction. However,
subtransactions can see rows from outer transactions, and once the
subtransaction completes, the outer transaction continues, requiring
the storage of all four fields. With subtransactions, an outer
transaction can create a row, a subtransaction expire it, and when the
subtransaction completes, the outer transaction still has to have
proper visibility of the row, for example, for cursors.
One possible solution is to create a phantom cid which represents a
cmin/cmax pair and is stored in local memory.
Diffstat (limited to 'doc/src')
| -rw-r--r-- | doc/src/FAQ/TODO.html | 21 |
1 files changed, 20 insertions, 1 deletions
diff --git a/doc/src/FAQ/TODO.html b/doc/src/FAQ/TODO.html index f24c9a5605f..d3cb0060893 100644 --- a/doc/src/FAQ/TODO.html +++ b/doc/src/FAQ/TODO.html @@ -8,7 +8,7 @@ <body bgcolor="#FFFFFF" text="#000000" link="#FF0000" vlink="#A00000" alink="#0000FF"> <h1><a name="section_1">PostgreSQL TODO List</a></h1> <p>Current maintainer: Bruce Momjian (<a href="mailto:pgman@candle.pha.pa.us">pgman@candle.pha.pa.us</a>)<br/> -Last updated: Thu Sep 1 20:49:44 EDT 2005 +Last updated: Fri Sep 2 11:22:36 EDT 2005 </p> <p>The most recent version of this document can be viewed at<br/> <a href="http://www.postgresql.org/docs/faqs.TODO.html">http://www.postgresql.org/docs/faqs.TODO.html</a>. @@ -867,6 +867,25 @@ first. could hit disk before WAL is written. </p> </li><li>Add a script to ask system configuration questions and tune postgresql.conf + </li><li>Merge xmin/xmax/cmin/cmax back into three header fields +<p> Before subtransactions, there used to be only three fields needed to + store these four values. This was possible because only the current + transaction looks at the cmin/cmax values. If the current transaction + created and expired the row the fields stored where xmin (same as + xmax), cmin, cmax, and if the transaction was expiring a row from a + another transaction, the fields stored were xmin (cmin was not + needed), xmax, and cmax. Such a system worked because a transaction + could only see committed rows from another transaction. However, + subtransactions can see rows from outer transactions, and once the + subtransaction completes, the outer transaction continues, requiring + the storage of all four fields. With subtransactions, an outer + transaction can create a row, a subtransaction expire it, and when the + subtransaction completes, the outer transaction still has to have + proper visibility of the row, for example, for cursors. +</p> +<p> One possible solution is to create a phantom cid which represents a + cmin/cmax pair and is stored in local memory. +</p> </li><li>Use a phantom command counter for nested subtransactions to reduce per-tuple overhead </li><li>Research storing disk pages with no alignment/padding |