Ignore extended statistics for inheritance trees

Since commit 859b3003de we only build extended statistics for individual
relations, ignoring the child relations. This resolved the issue with
updating catalog tuple twice, but we still tried to use the statistics
when calculating estimates for the whole inheritance tree. When the
relations contain very distinct data, it may produce bogus estimates.

This is roughly the same issue 427c6b5b9 addressed ~15 years ago, and we
fix it the same way - by ignoring extended statistics when calculating
estimates for the inheritance tree as a whole. We still consider
extended statistics when calculating estimates for individual child
relations, of course.

This may result in plan changes due to different estimates, but if the
old statistics were not describing the inheritance tree particularly
well it's quite likely the new plans is actually better.

Report and patch by Justin Pryzby, minor fixes and cleanup by me.
Backpatch all the way back to PostgreSQL 10, where extended statistics
were introduced (same as 859b3003de).

Author: Justin Pryzby
Reported-by: Justin Pryzby
Backpatch-through: 10
Discussion: https://postgr.es/m/20210923212624.GI831%40telsasoft.com

diff --git a/src/backend/statistics/dependencies.c b/src/backend/statistics/dependencies.c
index 8bf80db8e4d8e655eb73d7247da6c6cdcfa74e8d..a41bace75abe18986e63dc620aae0f1a9d2fd69d 100644 (file)
--- a/src/backend/statistics/dependencies.c
+++ b/src/backend/statistics/dependencies.c
@@ -24,6 +24,7 @@
 #include "nodes/pathnodes.h"
 #include "optimizer/clauses.h"
 #include "optimizer/optimizer.h"
+#include "parser/parsetree.h"
 #include "statistics/extended_stats_internal.h"
 #include "statistics/statistics.h"
 #include "utils/bytea.h"
@@ -1410,11 +1411,19 @@ dependencies_clauselist_selectivity(PlannerInfo *root,
    int         ndependencies;
    int         i;
    AttrNumber  attnum_offset;
+   RangeTblEntry *rte = planner_rt_fetch(rel->relid, root);
 
    /* unique expressions */
    Node      **unique_exprs;
    int         unique_exprs_cnt;
 
+   /*
+    * When dealing with inheritance trees, ignore extended stats (which were
+    * built without data from child rels, and thus do not represent them).
+    */
+   if (rte->inh)
+       return 1.0;
+
    /* check if there's any stats that might be useful for us. */
    if (!has_stats_of_kind(rel->statlist, STATS_EXT_DEPENDENCIES))
        return 1.0;

diff --git a/src/backend/statistics/extended_stats.c b/src/backend/statistics/extended_stats.c
index 0d106f0343fcab2cd4f327ec4216fa8ecdf6a307..366ae334f8b65a39e28bac77cc232513e95bb108 100644 (file)
--- a/src/backend/statistics/extended_stats.c
+++ b/src/backend/statistics/extended_stats.c
@@ -30,6 +30,7 @@
 #include "nodes/nodeFuncs.h"
 #include "optimizer/clauses.h"
 #include "optimizer/optimizer.h"
+#include "parser/parsetree.h"
 #include "pgstat.h"
 #include "postmaster/autovacuum.h"
 #include "statistics/extended_stats_internal.h"
@@ -1694,6 +1695,14 @@ statext_mcv_clauselist_selectivity(PlannerInfo *root, List *clauses, int varReli
    List      **list_exprs;     /* expressions matched to any statistic */
    int         listidx;
    Selectivity sel = (is_or) ? 0.0 : 1.0;
+   RangeTblEntry *rte = planner_rt_fetch(rel->relid, root);
+
+   /*
+    * When dealing with inheritance trees, ignore extended stats (which were
+    * built without data from child rels, and thus do not represent them).
+    */
+   if (rte->inh)
+       return sel;
 
    /* check if there's any stats that might be useful for us. */
    if (!has_stats_of_kind(rel->statlist, STATS_EXT_MCV))

diff --git a/src/backend/utils/adt/selfuncs.c b/src/backend/utils/adt/selfuncs.c
index 10895fb2876f8312527b6a4f7c9be08bcb7fdc38..1010d5caa86a3052666927e9acd04bda3e8d48f1 100644 (file)
--- a/src/backend/utils/adt/selfuncs.c
+++ b/src/backend/utils/adt/selfuncs.c
@@ -3913,6 +3913,14 @@ estimate_multivariate_ndistinct(PlannerInfo *root, RelOptInfo *rel,
    Oid         statOid = InvalidOid;
    MVNDistinct *stats;
    StatisticExtInfo *matched_info = NULL;
+   RangeTblEntry       *rte = planner_rt_fetch(rel->relid, root);
+
+   /*
+    * When dealing with inheritance trees, ignore extended stats (which were
+    * built without data from child rels, and thus do not represent them).
+    */
+   if (rte->inh)
+       return false;
 
    /* bail out immediately if the table has no extended statistics */
    if (!rel->statlist)
@@ -5222,6 +5230,7 @@ examine_variable(PlannerInfo *root, Node *node, int varRelid,
        foreach(slist, onerel->statlist)
        {
            StatisticExtInfo *info = (StatisticExtInfo *) lfirst(slist);
+           RangeTblEntry    *rte = planner_rt_fetch(onerel->relid, root);
            ListCell   *expr_item;
            int         pos;
 
@@ -5232,6 +5241,14 @@ examine_variable(PlannerInfo *root, Node *node, int varRelid,
            if (vardata->statsTuple)
                break;
 
+           /*
+            * When dealing with inheritance trees, ignore extended stats (which
+            * were built without data from child rels, and so do not represent
+            * them).
+            */
+           if (rte->inh)
+               break;
+
            /* skip stats without per-expression stats */
            if (info->kind != STATS_EXT_EXPRESSIONS)
                continue;

diff --git a/src/test/regress/expected/stats_ext.out b/src/test/regress/expected/stats_ext.out
index 2b97d7541b23b5da47b9e15f4ab1c32fe4ad9e68..a8f7f5f2f1289311660c76beaa3c2970210bb985 100644 (file)
--- a/src/test/regress/expected/stats_ext.out
+++ b/src/test/regress/expected/stats_ext.out
@@ -176,6 +176,47 @@ CREATE STATISTICS ab1_a_b_stats ON a, b FROM ab1;
 ANALYZE ab1;
 DROP TABLE ab1 CASCADE;
 NOTICE:  drop cascades to table ab1c
+-- Tests for stats with inheritance
+CREATE TABLE stxdinh(a int, b int);
+CREATE TABLE stxdinh1() INHERITS(stxdinh);
+CREATE TABLE stxdinh2() INHERITS(stxdinh);
+INSERT INTO stxdinh SELECT mod(a,50), mod(a,100) FROM generate_series(0, 1999) a;
+INSERT INTO stxdinh1 SELECT mod(a,100), mod(a,100) FROM generate_series(0, 999) a;
+INSERT INTO stxdinh2 SELECT mod(a,100), mod(a,100) FROM generate_series(0, 999) a;
+VACUUM ANALYZE stxdinh, stxdinh1, stxdinh2;
+-- Ensure non-inherited stats are not applied to inherited query
+-- Without stats object, it looks like this
+SELECT * FROM check_estimated_rows('SELECT a, b FROM stxdinh* GROUP BY 1, 2');
+ estimated | actual 
+-----------+--------
+       400 |    150
+(1 row)
+
+SELECT * FROM check_estimated_rows('SELECT a, b FROM stxdinh* WHERE a = 0 AND b = 0');
+ estimated | actual 
+-----------+--------
+         3 |     40
+(1 row)
+
+CREATE STATISTICS stxdinh ON a, b FROM stxdinh;
+VACUUM ANALYZE stxdinh, stxdinh1, stxdinh2;
+-- Since the stats object does not include inherited stats, it should not
+-- affect the estimates
+SELECT * FROM check_estimated_rows('SELECT a, b FROM stxdinh* GROUP BY 1, 2');
+ estimated | actual 
+-----------+--------
+       400 |    150
+(1 row)
+
+-- Dependencies are applied at individual relations (within append), so
+-- this estimate changes a bit because we improve estimates for the parent
+SELECT * FROM check_estimated_rows('SELECT a, b FROM stxdinh* WHERE a = 0 AND b = 0');
+ estimated | actual 
+-----------+--------
+        22 |     40
+(1 row)
+
+DROP TABLE stxdinh, stxdinh1, stxdinh2;
 -- basic test for statistics on expressions
 CREATE TABLE ab1 (a INTEGER, b INTEGER, c TIMESTAMP, d TIMESTAMPTZ);
 -- expression stats may be built on a single expression column

diff --git a/src/test/regress/sql/stats_ext.sql b/src/test/regress/sql/stats_ext.sql
index 6fb37962a720afc87c13f97fc7f7723f09df4ef1..69d7b71f0eb8731fad8a58b68826333855f8b200 100644 (file)
--- a/src/test/regress/sql/stats_ext.sql
+++ b/src/test/regress/sql/stats_ext.sql
@@ -112,6 +112,28 @@ CREATE STATISTICS ab1_a_b_stats ON a, b FROM ab1;
 ANALYZE ab1;
 DROP TABLE ab1 CASCADE;
 
+-- Tests for stats with inheritance
+CREATE TABLE stxdinh(a int, b int);
+CREATE TABLE stxdinh1() INHERITS(stxdinh);
+CREATE TABLE stxdinh2() INHERITS(stxdinh);
+INSERT INTO stxdinh SELECT mod(a,50), mod(a,100) FROM generate_series(0, 1999) a;
+INSERT INTO stxdinh1 SELECT mod(a,100), mod(a,100) FROM generate_series(0, 999) a;
+INSERT INTO stxdinh2 SELECT mod(a,100), mod(a,100) FROM generate_series(0, 999) a;
+VACUUM ANALYZE stxdinh, stxdinh1, stxdinh2;
+-- Ensure non-inherited stats are not applied to inherited query
+-- Without stats object, it looks like this
+SELECT * FROM check_estimated_rows('SELECT a, b FROM stxdinh* GROUP BY 1, 2');
+SELECT * FROM check_estimated_rows('SELECT a, b FROM stxdinh* WHERE a = 0 AND b = 0');
+CREATE STATISTICS stxdinh ON a, b FROM stxdinh;
+VACUUM ANALYZE stxdinh, stxdinh1, stxdinh2;
+-- Since the stats object does not include inherited stats, it should not
+-- affect the estimates
+SELECT * FROM check_estimated_rows('SELECT a, b FROM stxdinh* GROUP BY 1, 2');
+-- Dependencies are applied at individual relations (within append), so
+-- this estimate changes a bit because we improve estimates for the parent
+SELECT * FROM check_estimated_rows('SELECT a, b FROM stxdinh* WHERE a = 0 AND b = 0');
+DROP TABLE stxdinh, stxdinh1, stxdinh2;
+
 -- basic test for statistics on expressions
 CREATE TABLE ab1 (a INTEGER, b INTEGER, c TIMESTAMP, d TIMESTAMPTZ);