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

Also improve the comments a bit.

diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
index 8319f51e80ba391f2d77e5922bfa2ced669c6f54..5ca3c0241bc8860472942627cdc38320bde21fb3 100644 (file)
--- 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)