From 17d787a3b160eefb2ff4a3fdf12ca1fedc02cbc1 Mon Sep 17 00:00:00 2001
From: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date: Wed, 12 Mar 2014 20:43:07 +0200
Subject: [PATCH] Items on GIN data pages are no longer always 6 bytes; update
gincostestimate.
Also improve the comments a bit.
---
src/backend/utils/adt/selfuncs.c | 33 ++++++++++++++++----------------
1 file changed, 16 insertions(+), 17 deletions(-)
diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
index 8319f51e80b..5ca3c0241bc 100644
--- a/src/backend/utils/adt/selfuncs.c
+++ b/src/backend/utils/adt/selfuncs.c
@@ -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)
--
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