Identify simple column references in extended statistics

Until now, when defining extended statistics, everything except a plain
column reference was treated as complex expression. So for example "a"
was a column reference, but "(a)" would be an expression. In most cases
this does not matter much, but there were a couple strange consequences.
For example

    CREATE STATISTICS s ON a FROM t;

would fail, because extended stats require at least two columns. But

    CREATE STATISTICS s ON (a) FROM t;

would succeed, because that requirement does not apply to expressions.
Moreover, that statistics object is useless - the optimizer will always
use the regular statistics collected for attribute "a".

So do a bit more work to identify those expressions referencing a single
column, and translate them to a simple column reference. Backpatch to
14, where support for extended statistics on expressions was introduced.

Reported-by: Justin Pryzby
Backpatch-through: 14
Discussion: https://postgr.es/m/20210816013255.GS10479%40telsasoft.com

src/backend/commands/statscmds.c

@@ -33,6 +33,7 @@
 #include "optimizer/optimizer.h"
 #include "statistics/statistics.h"
 #include "utils/builtins.h"
+#include "utils/lsyscache.h"
 #include "utils/fmgroids.h"
 #include "utils/inval.h"
 #include "utils/memutils.h"
@@ -210,13 +211,15 @@ CreateStatistics(CreateStatsStmt *stmt)
 	/*
 	 * Convert the expression list to a simple array of attnums, but also keep
 	 * a list of more complex expressions.  While at it, enforce some
-	 * constraints.
+	 * constraints - we don't allow extended statistics on system attributes,
+	 * and we require the data type to have less-than operator.
 	 *
-	 * XXX We do only the bare minimum to separate simple attribute and
-	 * complex expressions - for example "(a)" will be treated as a complex
-	 * expression. No matter how elaborate the check is, there'll always be a
-	 * way around it, if the user is determined (consider e.g. "(a+0)"), so
-	 * it's not worth protecting against it.
+	 * There are many ways how to "mask" a simple attribute refenrece as an
+	 * expression, for example "(a+0)" etc. We can't possibly detect all of
+	 * them, but we handle at least the simple case with attribute in parens.
+	 * There'll always be a way around this, if the user is determined (like
+	 * the "(a+0)" example), but this makes it somewhat consistent with how
+	 * indexes treat attributes/expressions.
 	 */
 	foreach(cell, stmt->exprs)
 	{
@@ -257,6 +260,28 @@ CreateStatistics(CreateStatsStmt *stmt)
 			nattnums++;
 			ReleaseSysCache(atttuple);
 		}
+		else if (IsA(selem->expr, Var))	/* column reference in parens */
+		{
+			Var *var = (Var *) selem->expr;
+			TypeCacheEntry *type;
+
+			/* Disallow use of system attributes in extended stats */
+			if (var->varattno <= 0)
+				ereport(ERROR,
+						(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+						 errmsg("statistics creation on system columns is not supported")));
+
+			/* Disallow data types without a less-than operator */
+			type = lookup_type_cache(var->vartype, TYPECACHE_LT_OPR);
+			if (type->lt_opr == InvalidOid)
+				ereport(ERROR,
+						(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+						 errmsg("column \"%s\" cannot be used in statistics because its type %s has no default btree operator class",
+								get_attname(relid, var->varattno, false), format_type_be(var->vartype))));
+
+			attnums[nattnums] = var->varattno;
+			nattnums++;
+		}
 		else					/* expression */
 		{
 			Node	   *expr = selem->expr;

src/test/regress/expected/stats_ext.out

@@ -55,6 +55,8 @@ ERROR:  duplicate expression in statistics definition
 CREATE STATISTICS tst (unrecognized) ON x, y FROM ext_stats_test;
 ERROR:  unrecognized statistics kind "unrecognized"
 -- incorrect expressions
+CREATE STATISTICS tst ON (y) FROM ext_stats_test; -- single column reference
+ERROR:  extended statistics require at least 2 columns
 CREATE STATISTICS tst ON y + z FROM ext_stats_test; -- missing parentheses
 ERROR:  syntax error at or near "+"
 LINE 1: CREATE STATISTICS tst ON y + z FROM ext_stats_test;

src/test/regress/sql/stats_ext.sql

@@ -41,6 +41,7 @@ CREATE STATISTICS tst ON (x || 'x'), (x || 'x'), (y + 1), (x || 'x'), (x || 'x')
 CREATE STATISTICS tst ON (x || 'x'), (x || 'x'), y FROM ext_stats_test;
 CREATE STATISTICS tst (unrecognized) ON x, y FROM ext_stats_test;
 -- incorrect expressions
+CREATE STATISTICS tst ON (y) FROM ext_stats_test; -- single column reference
 CREATE STATISTICS tst ON y + z FROM ext_stats_test; -- missing parentheses
 CREATE STATISTICS tst ON (x, y) FROM ext_stats_test; -- tuple expression
 DROP TABLE ext_stats_test;