Fix tsmatchsel() to account properly for null rows.

ts_typanalyze.c computes MCE statistics as fractions of the non-null rows,
which seems fairly reasonable, and anyway changing it in released versions
wouldn't be a good idea.  But then ts_selfuncs.c has to account for that.
Failure to do so results in overestimates in columns with a significant
fraction of null documents.  Back-patch to 8.4 where this stuff was
introduced.

Jesper Krogh

1 files changed, 6 insertions, 0 deletions

diff --git a/src/backend/tsearch/ts_selfuncs.c b/src/backend/tsearch/ts_selfuncs.c
index 1f0a42d9b12..709d48c6178 100644
--- a/src/backend/tsearch/ts_selfuncs.c
+++ b/src/backend/tsearch/ts_selfuncs.c
@@ -188,11 +188,17 @@ tsquerysel(VariableStatData *vardata, Datum constval)
 			/* No most-common-elements info, so do without */
 			selec = tsquery_opr_selec_no_stats(query);
 		}
+
+		/*
+		 * MCE stats count only non-null rows, so adjust for null rows.
+		 */
+		selec *= (1.0 - stats->stanullfrac);
 	}
 	else
 	{
 		/* No stats at all, so do without */
 		selec = tsquery_opr_selec_no_stats(query);
+		/* we assume no nulls here, so no stanullfrac correction */
 	}
 
 	return selec;