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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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2
src/backend/utils/cache/relcache.c vendored
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@ -1414,6 +1414,7 @@ formrdesc(const char *relationName, Oid relationReltype,
relation->rd_rel->relpages = 0;
relation->rd_rel->reltuples = 0;
relation->rd_rel->relallvisible = 0;
relation->rd_rel->relkind = RELKIND_RELATION;
relation->rd_rel->relhasoids = hasoids;
relation->rd_rel->relnatts = (int16) natts;
@ -2668,6 +2669,7 @@ RelationSetNewRelfilenode(Relation relation, TransactionId freezeXid)
{
classform->relpages = 0; /* it's empty until further notice */
classform->reltuples = 0;
classform->relallvisible = 0;
}
classform->relfrozenxid = freezeXid;