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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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68
src/backend/access/heap/visibilitymap.c
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@ -16,6 +16,8 @@
* visibilitymap_pin_ok - check whether correct map page is already pinned
* visibilitymap_set - set a bit in a previously pinned page
* visibilitymap_test - test if a bit is set
* visibilitymap_count - count number of bits set in visibility map
* visibilitymap_truncate - truncate the visibility map
*
* NOTES
*
@ -110,6 +112,26 @@
#define HEAPBLK_TO_MAPBYTE(x) (((x) % HEAPBLOCKS_PER_PAGE) / HEAPBLOCKS_PER_BYTE)
#define HEAPBLK_TO_MAPBIT(x) ((x) % HEAPBLOCKS_PER_BYTE)
/* table for fast counting of set bits */
static const uint8 number_of_ones[256] = {
0, 1, 1, 2, 1, 2, 2, 3, 1, 2, 2, 3, 2, 3, 3, 4,
1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5,
1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5,
2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6,
1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5,
2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6,
2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6,
3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7,
1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5,
2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6,
2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6,
3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7,
2, 3, 3, 4, 3, 4, 4, 5, 3, 4, 4, 5, 4, 5, 5, 6,
3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7,
3, 4, 4, 5, 4, 5, 5, 6, 4, 5, 5, 6, 5, 6, 6, 7,
4, 5, 5, 6, 5, 6, 6, 7, 5, 6, 6, 7, 6, 7, 7, 8
};
/* prototypes for internal routines */
static Buffer vm_readbuf(Relation rel, BlockNumber blkno, bool extend);
static void vm_extend(Relation rel, BlockNumber nvmblocks);
@ -307,6 +329,52 @@ visibilitymap_test(Relation rel, BlockNumber heapBlk, Buffer *buf)
return result;
}
/*
* visibilitymap_count - count number of bits set in visibility map
*
* Note: we ignore the possibility of race conditions when the table is being
* extended concurrently with the call. New pages added to the table aren't
* going to be marked all-visible, so they won't affect the result.
*/
BlockNumber
visibilitymap_count(Relation rel)
{
BlockNumber result = 0;
BlockNumber mapBlock;
for (mapBlock = 0; ; mapBlock++)
{
Buffer mapBuffer;
unsigned char *map;
int i;
/*
* Read till we fall off the end of the map. We assume that any
* extra bytes in the last page are zeroed, so we don't bother
* excluding them from the count.
*/
mapBuffer = vm_readbuf(rel, mapBlock, false);
if (!BufferIsValid(mapBuffer))
break;
/*
* We choose not to lock the page, since the result is going to be
* immediately stale anyway if anyone is concurrently setting or
* clearing bits, and we only really need an approximate value.
*/
map = (unsigned char *) PageGetContents(BufferGetPage(mapBuffer));
for (i = 0; i < MAPSIZE; i++)
{
result += number_of_ones[map[i]];
}
ReleaseBuffer(mapBuffer);
}
return result;
}
/*
* visibilitymap_truncate - truncate the visibility map
*