@@ -7291,31 +7291,30 @@ gincostestimate(PG_FUNCTION_ARGS)
72917291* indexStartupCost = (entryPagesFetched + dataPagesFetched )* spc_random_page_cost ;
72927292
72937293/*
7294- * Now we compute the number of data pages fetched while the scan
7295- * proceeds.
7294+ * Now compute the number of data pages fetched during the scan.
7295+ *
7296+ * We assume every entry to have the same number of items, and that there
7297+ * is no overlap between them. (XXX: tsvector and array opclasses collect
7298+ * statistics on the frequency of individual keys; it would be nice to
7299+ * use those here.)
72967300 */
7297-
7298- /* data pages scanned for each exact (non-partial) matched entry */
72997301dataPagesFetched = ceil (numDataPages * counts .exactEntries /numEntries );
73007302
73017303/*
7302- * Estimate number of data pages read, using selectivity estimation and
7303- * capacity of data page.
7304+ * If there is a lot of overlap among the entries, in particular if one
7305+ * of the entries is very frequent, the above calculation can grossly
7306+ * under-estimate. As a simple cross-check, calculate a lower bound
7307+ * based on the overall selectivity of the quals. At a minimum, we must
7308+ * read one item pointer for each matching entry.
7309+ *
7310+ * The width of each item pointer varies, based on the level of
7311+ * compression. We don't have statistics on that, but an average of
7312+ * around 3 bytes per item is fairly typical.
73047313 */
73057314dataPagesFetchedBySel = ceil (* indexSelectivity *
7306- (numTuples / (BLCKSZ /SizeOfIptrData )));
7307-
7315+ (numTuples / (BLCKSZ /3 )));
73087316if (dataPagesFetchedBySel > dataPagesFetched )
7309- {
7310- /*
7311- * At least one of entries is very frequent and, unfortunately, we
7312- * couldn't get statistic about entries (only tsvector has such
7313- * statistics). So, we obviously have too small estimation of pages
7314- * fetched from data tree. Re-estimate it from known capacity of data
7315- * pages
7316- */
73177317dataPagesFetched = dataPagesFetchedBySel ;
7318- }
73197318
73207319/* Account for cache effects, the same as above */
73217320if (outer_scans > 1 || counts .arrayScans > 1 )