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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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20
src/backend/commands/vacuumlazy.c
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@ -158,6 +158,7 @@ lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,
TransactionId freezeTableLimit;
BlockNumber new_rel_pages;
double new_rel_tuples;
BlockNumber new_rel_allvisible;
TransactionId new_frozen_xid;
/* measure elapsed time iff autovacuum logging requires it */
@ -222,6 +223,10 @@ lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,
* density") with nonzero relpages and reltuples=0 (which means "zero
* tuple density") unless there's some actual evidence for the latter.
*
* We do update relallvisible even in the corner case, since if the
* table is all-visible we'd definitely like to know that. But clamp
* the value to be not more than what we're setting relpages to.
*
* Also, don't change relfrozenxid if we skipped any pages, since then
* we don't know for certain that all tuples have a newer xmin.
*/
@ -233,12 +238,18 @@ lazy_vacuum_rel(Relation onerel, VacuumStmt *vacstmt,
new_rel_tuples = vacrelstats->old_rel_tuples;
}
new_rel_allvisible = visibilitymap_count(onerel);
if (new_rel_allvisible > new_rel_pages)
new_rel_allvisible = new_rel_pages;
new_frozen_xid = FreezeLimit;
if (vacrelstats->scanned_pages < vacrelstats->rel_pages)
new_frozen_xid = InvalidTransactionId;
vac_update_relstats(onerel,
new_rel_pages, new_rel_tuples,
new_rel_pages,
new_rel_tuples,
new_rel_allvisible,
vacrelstats->hasindex,
new_frozen_xid);
@ -1063,8 +1074,11 @@ lazy_cleanup_index(Relation indrel,
*/
if (!stats->estimated_count)
vac_update_relstats(indrel,
stats->num_pages, stats->num_index_tuples,
false, InvalidTransactionId);
stats->num_pages,
stats->num_index_tuples,
0,
false,
InvalidTransactionId);
ereport(elevel,
(errmsg("index \"%s\" now contains %.0f row versions in %u pages",