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 82b549fdc2cd9503fa2eb6985b445c1e533bae0c..f1602cd3a261166781fd764f04c1ad8880ead7c6 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 4ab9aaa591c246b8c7e122ec85d2f738f6e13d0e..b23c15e9db4a8244a049ed3113966c631d4fc972 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 aff748d67ba58d23839dc8eb5019c195ba5792e4..9e39f9888e1f12a5ad20e915dbc382288d8636e2 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 c60ba45aba82b8e7db4391a6306ec9de8d095b3f..abe1f89edd86da574de4bc8fe855107623070113 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);