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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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src/include/access/visibilitymap.h
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@@ -27,6 +27,7 @@ extern bool visibilitymap_pin_ok(BlockNumber heapBlk, Buffer vmbuf);
extern void visibilitymap_set(Relation rel, BlockNumber heapBlk,
XLogRecPtr recptr, Buffer vmbuf);
extern bool visibilitymap_test(Relation rel, BlockNumber heapBlk, Buffer *vmbuf);
extern void visibilitymap_truncate(Relation rel, BlockNumber heapblk);
extern BlockNumber visibilitymap_count(Relation rel);
extern void visibilitymap_truncate(Relation rel, BlockNumber nheapblocks);
#endif /* VISIBILITYMAP_H */