Disallow extended statistics on system columns

Since introduction of extended statistics, we've disallowed references
to system columns. So for example

    CREATE STATISTICS s ON ctid FROM t;

would fail. But with extended statistics on expressions, it was possible
to work around this limitation quite easily

    CREATE STATISTICS s ON (ctid::text) FROM t;

This is an oversight in a4d75c86bf, fixed by adding a simple check.
Backpatch to PostgreSQL 14, where support for extended statistics on
expressions was introduced.

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

diff --git a/src/backend/commands/statscmds.c b/src/backend/commands/statscmds.c
index 78917844deee8cc3c3d1df2d1ddbdf82160d9ba5..afe6744e237010c2d9482fa5eb5db88a95836322 100644 (file)
--- a/src/backend/commands/statscmds.c
+++ b/src/backend/commands/statscmds.c
@@ -288,9 +288,24 @@ CreateStatistics(CreateStatsStmt *stmt)
            Node       *expr = selem->expr;
            Oid         atttype;
            TypeCacheEntry *type;
+           Bitmapset  *attnums = NULL;
+           int         k;
 
            Assert(expr != NULL);
 
+           /* Disallow expressions referencing system attributes. */
+           pull_varattnos(expr, 1, &attnums);
+
+           k = -1;
+           while ((k = bms_next_member(attnums, k)) >= 0)
+           {
+               AttrNumber  attnum = k + FirstLowInvalidHeapAttributeNumber;
+               if (attnum <= 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.
             *