Extended statistics on expressions

Allow defining extended statistics on expressions, not just just on
simple column references.  With this commit, expressions are supported
by all existing extended statistics kinds, improving the same types of
estimates. A simple example may look like this:

  CREATE TABLE t (a int);
  CREATE STATISTICS s ON mod(a,10), mod(a,20) FROM t;
  ANALYZE t;

The collected statistics are useful e.g. to estimate queries with those
expressions in WHERE or GROUP BY clauses:

  SELECT * FROM t WHERE mod(a,10) = 0 AND mod(a,20) = 0;

  SELECT 1 FROM t GROUP BY mod(a,10), mod(a,20);

This introduces new internal statistics kind 'e' (expressions) which is
built automatically when the statistics object definition includes any
expressions. This represents single-expression statistics, as if there
was an expression index (but without the index maintenance overhead).
The statistics is stored in pg_statistics_ext_data as an array of
composite types, which is possible thanks to 79f6a942bd.

CREATE STATISTICS allows building statistics on a single expression, in
which case in which case it's not possible to specify statistics kinds.

A new system view pg_stats_ext_exprs can be used to display expression
statistics, similarly to pg_stats and pg_stats_ext views.

ALTER TABLE ... ALTER COLUMN ... TYPE now treats indexes the same way it
treats indexes, i.e. it drops and recreates the statistics. This means
all statistics are reset, and we no longer try to preserve at least the
functional dependencies. This should not be a major issue in practice,
as the functional dependencies actually rely on per-column statistics,
which were always reset anyway.

Author: Tomas Vondra
Reviewed-by: Justin Pryzby, Dean Rasheed, Zhihong Yu
Discussion: https://postgr.es/m/ad7891d2-e90c-b446-9fe2-7419143847d7%40enterprisedb.com

1 files changed, 62 insertions, 0 deletions

diff --git a/src/backend/optimizer/util/plancat.c b/src/backend/optimizer/util/plancat.c
index 6c39bf893f8..0fa8875f091 100644
--- a/src/backend/optimizer/util/plancat.c
+++ b/src/backend/optimizer/util/plancat.c
@@ -34,6 +34,7 @@
 #include "foreign/fdwapi.h"
 #include "miscadmin.h"
 #include "nodes/makefuncs.h"
+#include "nodes/nodeFuncs.h"
 #include "nodes/supportnodes.h"
 #include "optimizer/clauses.h"
 #include "optimizer/cost.h"
@@ -1308,6 +1309,7 @@ get_relation_constraints(PlannerInfo *root,
 static List *
 get_relation_statistics(RelOptInfo *rel, Relation relation)
 {
+	Index		varno = rel->relid;
 	List	   *statoidlist;
 	List	   *stainfos = NIL;
 	ListCell   *l;
@@ -1321,6 +1323,7 @@ get_relation_statistics(RelOptInfo *rel, Relation relation)
 		HeapTuple	htup;
 		HeapTuple	dtup;
 		Bitmapset  *keys = NULL;
+		List	   *exprs = NIL;
 		int			i;
 
 		htup = SearchSysCache1(STATEXTOID, ObjectIdGetDatum(statOid));
@@ -1340,6 +1343,49 @@ get_relation_statistics(RelOptInfo *rel, Relation relation)
 		for (i = 0; i < staForm->stxkeys.dim1; i++)
 			keys = bms_add_member(keys, staForm->stxkeys.values[i]);
 
+		/*
+		 * Preprocess expressions (if any). We read the expressions, run them
+		 * through eval_const_expressions, and fix the varnos.
+		 */
+		{
+			bool		isnull;
+			Datum		datum;
+
+			/* decode expression (if any) */
+			datum = SysCacheGetAttr(STATEXTOID, htup,
+									Anum_pg_statistic_ext_stxexprs, &isnull);
+
+			if (!isnull)
+			{
+				char	   *exprsString;
+
+				exprsString = TextDatumGetCString(datum);
+				exprs = (List *) stringToNode(exprsString);
+				pfree(exprsString);
+
+				/*
+				 * Run the expressions through eval_const_expressions. This is
+				 * not just an optimization, but is necessary, because the
+				 * planner will be comparing them to similarly-processed qual
+				 * clauses, and may fail to detect valid matches without this.
+				 * We must not use canonicalize_qual, however, since these
+				 * aren't qual expressions.
+				 */
+				exprs = (List *) eval_const_expressions(NULL, (Node *) exprs);
+
+				/* May as well fix opfuncids too */
+				fix_opfuncids((Node *) exprs);
+
+				/*
+				 * Modify the copies we obtain from the relcache to have the
+				 * correct varno for the parent relation, so that they match
+				 * up correctly against qual clauses.
+				 */
+				if (varno != 1)
+					ChangeVarNodes((Node *) exprs, 1, varno, 0);
+			}
+		}
+
 		/* add one StatisticExtInfo for each kind built */
 		if (statext_is_kind_built(dtup, STATS_EXT_NDISTINCT))
 		{
@@ -1349,6 +1395,7 @@ get_relation_statistics(RelOptInfo *rel, Relation relation)
 			info->rel = rel;
 			info->kind = STATS_EXT_NDISTINCT;
 			info->keys = bms_copy(keys);
+			info->exprs = exprs;
 
 			stainfos = lappend(stainfos, info);
 		}
@@ -1361,6 +1408,7 @@ get_relation_statistics(RelOptInfo *rel, Relation relation)
 			info->rel = rel;
 			info->kind = STATS_EXT_DEPENDENCIES;
 			info->keys = bms_copy(keys);
+			info->exprs = exprs;
 
 			stainfos = lappend(stainfos, info);
 		}
@@ -1373,6 +1421,20 @@ get_relation_statistics(RelOptInfo *rel, Relation relation)
 			info->rel = rel;
 			info->kind = STATS_EXT_MCV;
 			info->keys = bms_copy(keys);
+			info->exprs = exprs;
+
+			stainfos = lappend(stainfos, info);
+		}
+
+		if (statext_is_kind_built(dtup, STATS_EXT_EXPRESSIONS))
+		{
+			StatisticExtInfo *info = makeNode(StatisticExtInfo);
+
+			info->statOid = statOid;
+			info->rel = rel;
+			info->kind = STATS_EXT_EXPRESSIONS;
+			info->keys = bms_copy(keys);
+			info->exprs = exprs;
 
 			stainfos = lappend(stainfos, info);
 		}