diff options
Diffstat (limited to 'src/backend/access/hash/README')
| -rw-r--r-- | src/backend/access/hash/README | 18 |
1 files changed, 17 insertions, 1 deletions
diff --git a/src/backend/access/hash/README b/src/backend/access/hash/README index ce195eae2cd..0e1a9ed1229 100644 --- a/src/backend/access/hash/README +++ b/src/backend/access/hash/README @@ -1,4 +1,4 @@ -$Header: /cvsroot/pgsql/src/backend/access/hash/README,v 1.3 2003/09/04 22:06:27 tgl Exp $ +$Header: /cvsroot/pgsql/src/backend/access/hash/README,v 1.3.2.1 2007/04/19 20:24:36 tgl Exp $ This directory contains an implementation of hash indexing for Postgres. @@ -72,6 +72,18 @@ index, and preparing to allocate additional overflow pages after those bucket pages. hashm_spares[] entries before S cannot change anymore, since that would require moving already-created bucket pages. +The last page nominally used by the index is always determinable from +hashm_spares[S]. To avoid complaints from smgr, the logical EOF as seen by +the filesystem and smgr must always be greater than or equal to this page. +We have to allow the case "greater than" because it's possible that during +an index extension we crash after allocating filesystem space and before +updating the metapage. Note that on filesystems that allow "holes" in +files, it's entirely likely that pages before the logical EOF are not yet +allocated: when we allocate a new splitpoint's worth of bucket pages, we +physically zero the last such page to force the EOF up, and the first such +page will be used immediately, but the intervening pages are not written +until needed. + Since overflow pages may be recycled if enough tuples are deleted from their bucket, we need a way to keep track of currently-free overflow pages. The state of each overflow page (0 = available, 1 = not available) @@ -305,6 +317,10 @@ we can just error out without any great harm being done. Free space management --------------------- +(Question: why is this so complicated? Why not just have a linked list +of free pages with the list head in the metapage? It's not like we +avoid needing to modify the metapage with all this.) + Free space management consists of two sub-algorithms, one for reserving an overflow page to add to a bucket chain, and one for returning an empty overflow page to the free pool. |