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Measure the number of all-visible pages for use in index-only scan costing.

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Add a column pg_class.relallvisible to remember the number of pages that
were all-visible according to the visibility map as of the last VACUUM
(or ANALYZE, or some other operations that update pg_class.relpages).
Use relallvisible/relpages, instead of an arbitrary constant, to estimate
how many heap page fetches can be avoided during an index-only scan.

This is pretty primitive and will no doubt see refinements once we've
acquired more field experience with the index-only scan mechanism, but
it's way better than using a constant.

Note: I had to adjust an underspecified query in the window.sql regression
test, because it was changing answers when the plan changed to use an
index-only scan.  Some of the adjacent tests perhaps should be adjusted
as well, but I didn't do that here.
This commit is contained in:
Tom Lane
2011-10-14 17:23:01 -04:00
parent dea95c7a7b
commit e6858e6657
22 changed files with 246 additions and 72 deletions
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52
src/backend/catalog/index.c
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@@ -26,6 +26,7 @@
#include "access/relscan.h"
#include "access/sysattr.h"
#include "access/transam.h"
#include "access/visibilitymap.h"
#include "access/xact.h"
#include "bootstrap/bootstrap.h"
#include "catalog/catalog.h"
@@ -1059,7 +1060,7 @@ index_create(Relation heapRelation,
true,
isprimary,
InvalidOid,
heapRelation->rd_rel->reltuples);
-1.0);
/* Make the above update visible */
CommandCounterIncrement();
}
@@ -1225,7 +1226,7 @@ index_constraint_create(Relation heapRelation,
true,
true,
InvalidOid,
heapRelation->rd_rel->reltuples);
-1.0);
/*
* If needed, mark the index as primary and/or deferred in pg_index.
@@ -1533,9 +1534,10 @@ FormIndexDatum(IndexInfo *indexInfo,
* isprimary: if true, set relhaspkey true; else no change
* reltoastidxid: if not InvalidOid, set reltoastidxid to this value;
* else no change
* reltuples: set reltuples to this value
* reltuples: if >= 0, set reltuples to this value; else no change
*
* relpages is also updated (using RelationGetNumberOfBlocks()).
* If reltuples >= 0, relpages and relallvisible are also updated (using
* RelationGetNumberOfBlocks() and visibilitymap_count()).
*
* NOTE: an important side-effect of this operation is that an SI invalidation
* message is sent out to all backends --- including me --- causing relcache
@@ -1550,7 +1552,6 @@ index_update_stats(Relation rel,
bool hasindex, bool isprimary,
Oid reltoastidxid, double reltuples)
{
BlockNumber relpages = RelationGetNumberOfBlocks(rel);
Oid relid = RelationGetRelid(rel);
Relation pg_class;
HeapTuple tuple;
@@ -1586,9 +1587,11 @@ index_update_stats(Relation rel,
* It is safe to use a non-transactional update even though our
* transaction could still fail before committing. Setting relhasindex
* true is safe even if there are no indexes (VACUUM will eventually fix
* it), likewise for relhaspkey. And of course the relpages and reltuples
* counts are correct (or at least more so than the old values)
* regardless.
* it), likewise for relhaspkey. And of course the new relpages and
* reltuples counts are correct regardless. However, we don't want to
* change relpages (or relallvisible) if the caller isn't providing an
* updated reltuples count, because that would bollix the
* reltuples/relpages ratio which is what's really important.
*/
pg_class = heap_open(RelationRelationId, RowExclusiveLock);
@@ -1650,15 +1653,32 @@ index_update_stats(Relation rel,
dirty = true;
}
}
if (rd_rel->reltuples != (float4) reltuples)
if (reltuples >= 0)
{
rd_rel->reltuples = (float4) reltuples;
dirty = true;
}
if (rd_rel->relpages != (int32) relpages)
{
rd_rel->relpages = (int32) relpages;
dirty = true;
BlockNumber relpages = RelationGetNumberOfBlocks(rel);
BlockNumber relallvisible;
if (rd_rel->relkind != RELKIND_INDEX)
relallvisible = visibilitymap_count(rel);
else /* don't bother for indexes */
relallvisible = 0;
if (rd_rel->relpages != (int32) relpages)
{
rd_rel->relpages = (int32) relpages;
dirty = true;
}
if (rd_rel->reltuples != (float4) reltuples)
{
rd_rel->reltuples = (float4) reltuples;
dirty = true;
}
if (rd_rel->relallvisible != (int32) relallvisible)
{
rd_rel->relallvisible = (int32) relallvisible;
dirty = true;
}
}
/*