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Items on GIN data pages are no longer always 6 bytes; update gincostestimate.

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Also improve the comments a bit.
This commit is contained in:
Heikki Linnakangas 2014-03-12 20:43:07 +02:00
parent 588fb50715
commit 17d787a3b1
1 changed files with 16 additions and 17 deletions
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33
src/backend/utils/adt/selfuncs.c
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@ -7291,31 +7291,30 @@ gincostestimate(PG_FUNCTION_ARGS)
*indexStartupCost = (entryPagesFetched + dataPagesFetched) * spc_random_page_cost;
/*
* Now we compute the number of data pages fetched while the scan
* proceeds.
* Now compute the number of data pages fetched during the scan.
*
* We assume every entry to have the same number of items, and that there
* is no overlap between them. (XXX: tsvector and array opclasses collect
* statistics on the frequency of individual keys; it would be nice to
* use those here.)
*/
/* data pages scanned for each exact (non-partial) matched entry */
dataPagesFetched = ceil(numDataPages * counts.exactEntries / numEntries);
/*
* Estimate number of data pages read, using selectivity estimation and
* capacity of data page.
* If there is a lot of overlap among the entries, in particular if one
* of the entries is very frequent, the above calculation can grossly
* under-estimate. As a simple cross-check, calculate a lower bound
* based on the overall selectivity of the quals. At a minimum, we must
* read one item pointer for each matching entry.
*
* The width of each item pointer varies, based on the level of
* compression. We don't have statistics on that, but an average of
* around 3 bytes per item is fairly typical.
*/
dataPagesFetchedBySel = ceil(*indexSelectivity *
(numTuples / (BLCKSZ / SizeOfIptrData)));
(numTuples / (BLCKSZ / 3)));
if (dataPagesFetchedBySel > dataPagesFetched)
{
/*
* At least one of entries is very frequent and, unfortunately, we
* couldn't get statistic about entries (only tsvector has such
* statistics). So, we obviously have too small estimation of pages
* fetched from data tree. Re-estimate it from known capacity of data
* pages
*/
dataPagesFetched = dataPagesFetchedBySel;
}
/* Account for cache effects, the same as above */
if (outer_scans > 1 || counts.arrayScans > 1)