From 52b60530f257b1591d8b72264cd6c0dd9aabfd46 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Thu, 17 Feb 2011 19:00:49 -0500
Subject: [PATCH] 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
---
src/backend/tsearch/ts_selfuncs.c | 6 ++++++
src/include/catalog/pg_statistic.h | 2 ++
2 files changed, 8 insertions(+)
diff --git a/src/backend/tsearch/ts_selfuncs.c b/src/backend/tsearch/ts_selfuncs.c
index 8ce9fb46aa1..7f33c16a245 100644
--- a/src/backend/tsearch/ts_selfuncs.c
+++ b/src/backend/tsearch/ts_selfuncs.c
@@ -189,11 +189,17 @@ 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;
diff --git a/src/include/catalog/pg_statistic.h b/src/include/catalog/pg_statistic.h
index f38921f1c69..927cd0b0471 100644
--- a/src/include/catalog/pg_statistic.h
+++ b/src/include/catalog/pg_statistic.h
@@ -246,6 +246,8 @@ typedef FormData_pg_statistic *Form_pg_statistic;
* type with identifiable elements (for instance, tsvector). staop contains
* the equality operator appropriate to the element type. stavalues contains
* the most common element values, and stanumbers their frequencies. Unlike
+ * MCV slots, frequencies are measured as the fraction of non-null rows the
+ * element value appears in, not the frequency of all rows. Also unlike
* MCV slots, the values are sorted into order (to support binary search
* for a particular value). Since this puts the minimum and maximum
* frequencies at unpredictable spots in stanumbers, there are two extra
--
GitLab