Reduce lock level for CREATE STATISTICS

In line with other lock reductions related to planning.

Simon Riggs

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 &&