Block ALTER INDEX/TABLE index_name ALTER COLUMN colname SET (options)

The grammar of this command run on indexes with column names has always
been authorized by the parser, and it has never been documented.

Since 911e7020, it is possible to define opclass parameters as of CREATE
INDEX, which actually broke the old case of ALTER INDEX/TABLE where
relation-level parameters n_distinct and n_distinct_inherited could be
defined for an index (see 76a47c0e and its thread where this point has
been touched, still remained unused).  Attempting to do that in v13~
would cause the index to become unusable, as there is a new dedicated
code path to load opclass parameters instead of the relation-level ones
previously available.  Note that it is possible to fix things with a
manual catalog update to bring the relation back online.

This commit disables this command for now as the use of column names for
indexes does not make sense anyway, particularly when it comes to index
expressions where names are automatically computed.  One way to properly
support this case properly in the future would be to use column numbers
when it comes to indexes, in the same way as ALTER INDEX .. ALTER COLUMN
.. SET STATISTICS.

Partitioned indexes were already blocked, but not indexes.  Some tests
are added for both cases.

There was some code in ANALYZE to enforce n_distinct to be used for an
index expression if the parameter was defined, but just remove it for
now until/if there is support for this (note that index-level parameters
never had support in pg_dump either, previously), so this was just dead
code.

Reported-by: Matthijs van der Vleuten
Author: Nathan Bossart, Michael Paquier
Reviewed-by: Vik Fearing, Dilip Kumar
Discussion: https://postgr.es/m/17220-15d684c6c2171a83@postgresql.org
Backpatch-through: 13

src/backend/commands/analyze.c

@@ -965,9 +965,6 @@ compute_index_stats(Relation onerel, double totalrows,
 			for (i = 0; i < attr_cnt; i++)
 			{
 				VacAttrStats *stats = thisdata->vacattrstats[i];
-				AttributeOpts *aopt =
-				get_attribute_options(stats->attr->attrelid,
-									  stats->attr->attnum);
 
 				stats->exprvals = exprvals + i;
 				stats->exprnulls = exprnulls + i;
@@ -977,14 +974,6 @@ compute_index_stats(Relation onerel, double totalrows,
 									 numindexrows,
 									 totalindexrows);
 
-				/*
-				 * If the n_distinct option is specified, it overrides the
-				 * above computation.  For indices, we always use just
-				 * n_distinct, not n_distinct_inherited.
-				 */
-				if (aopt != NULL && aopt->n_distinct != 0.0)
-					stats->stadistinct = aopt->n_distinct;
-
 				MemoryContextResetAndDeleteChildren(col_context);
 			}
 		}

src/backend/commands/tablecmds.c

@@ -4491,7 +4491,7 @@ ATPrepCmd(List **wqueue, Relation rel, AlterTableCmd *cmd,
 			break;
 		case AT_SetOptions:		/* ALTER COLUMN SET ( options ) */
 		case AT_ResetOptions:	/* ALTER COLUMN RESET ( options ) */
-			ATSimplePermissions(rel, ATT_TABLE | ATT_MATVIEW | ATT_INDEX | ATT_FOREIGN_TABLE);
+			ATSimplePermissions(rel, ATT_TABLE | ATT_MATVIEW | ATT_FOREIGN_TABLE);
 			/* This command never recurses */
 			pass = AT_PASS_MISC;
 			break;

src/test/regress/expected/btree_index.out

@@ -329,3 +329,15 @@ INSERT INTO delete_test_table SELECT i, 1, 2, 3 FROM generate_series(1,1000) i;
 -- Test unsupported btree opclass parameters
 create index on btree_tall_tbl (id int4_ops(foo=1));
 ERROR:  operator class int4_ops has no options
+-- Test case of ALTER INDEX with abuse of column names for indexes.
+-- This grammar is not officially supported, but the parser allows it.
+CREATE INDEX btree_tall_idx2 ON btree_tall_tbl (id);
+ALTER INDEX btree_tall_idx2 ALTER COLUMN id SET (n_distinct=100);
+ERROR:  "btree_tall_idx2" is not a table, materialized view, or foreign table
+DROP INDEX btree_tall_idx2;
+-- Partitioned index
+CREATE TABLE btree_part (id int4) PARTITION BY RANGE (id);
+CREATE INDEX btree_part_idx ON btree_part(id);
+ALTER INDEX btree_part_idx ALTER COLUMN id SET (n_distinct=100);
+ERROR:  "btree_part_idx" is not a table, materialized view, or foreign table
+DROP TABLE btree_part;

src/test/regress/sql/btree_index.sql

@@ -172,3 +172,14 @@ INSERT INTO delete_test_table SELECT i, 1, 2, 3 FROM generate_series(1,1000) i;
 
 -- Test unsupported btree opclass parameters
 create index on btree_tall_tbl (id int4_ops(foo=1));
+
+-- Test case of ALTER INDEX with abuse of column names for indexes.
+-- This grammar is not officially supported, but the parser allows it.
+CREATE INDEX btree_tall_idx2 ON btree_tall_tbl (id);
+ALTER INDEX btree_tall_idx2 ALTER COLUMN id SET (n_distinct=100);
+DROP INDEX btree_tall_idx2;
+-- Partitioned index
+CREATE TABLE btree_part (id int4) PARTITION BY RANGE (id);
+CREATE INDEX btree_part_idx ON btree_part(id);
+ALTER INDEX btree_part_idx ALTER COLUMN id SET (n_distinct=100);
+DROP TABLE btree_part;