Reduce lock level for CREATE STATISTICS

In line with other lock reductions related to planning.

Simon Riggs

Author: simonat2ndQuadrant

doc/src/sgml/mvcc.sgml

@@ -923,7 +923,8 @@ ERROR:  could not serialize access due to read/write dependencies among transact
 
         <para>
          Acquired by <command>VACUUM</command> (without <option>FULL</option>),
-         <command>ANALYZE</>, <command>CREATE INDEX CONCURRENTLY</>, and
+         <command>ANALYZE</>, <command>CREATE INDEX CONCURRENTLY</>,
+         <command>CREATE STATISTICS</> and
          <command>ALTER TABLE VALIDATE</command> and other
          <command>ALTER TABLE</command> variants (for full details see
          <xref linkend="SQL-ALTERTABLE">).

src/backend/commands/statscmds.c

@@ -96,7 +96,13 @@ CreateStatistics(CreateStatsStmt *stmt)
 				 errmsg("statistics \"%s\" already exist", namestr)));
 	}
 
-	rel = heap_openrv(stmt->relation, AccessExclusiveLock);
+	/*
+	 * CREATE STATISTICS will influence future execution plans but does
+	 * not interfere with currently executing plans so it is safe to
+	 * take only ShareUpdateExclusiveLock on relation, conflicting with
+	 * ANALYZE and other DDL that sets statistical information.
+	 */
+	rel = heap_openrv(stmt->relation, ShareUpdateExclusiveLock);
 	relid = RelationGetRelid(rel);
 
 	if (rel->rd_rel->relkind != RELKIND_RELATION &&