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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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3
src/backend/access/hash/hash.c
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@@ -55,6 +55,7 @@ hashbuild(PG_FUNCTION_ARGS)
IndexBuildResult *result;
BlockNumber relpages;
double reltuples;
double allvisfrac;
uint32 num_buckets;
HashBuildState buildstate;
@@ -67,7 +68,7 @@ hashbuild(PG_FUNCTION_ARGS)
RelationGetRelationName(index));
/* Estimate the number of rows currently present in the table */
estimate_rel_size(heap, NULL, &relpages, &reltuples);
estimate_rel_size(heap, NULL, &relpages, &reltuples, &allvisfrac);
/* Initialize the hash index metadata page and initial buckets */
num_buckets = _hash_metapinit(index, reltuples, MAIN_FORKNUM);