diff options
| -rw-r--r-- | doc/src/sgml/release-9.1.sgml | 17 | ||||
| -rw-r--r-- | src/backend/commands/tablecmds.c | 28 |
2 files changed, 29 insertions, 16 deletions
diff --git a/doc/src/sgml/release-9.1.sgml b/doc/src/sgml/release-9.1.sgml index 9dfca1c7866..914464eb6c3 100644 --- a/doc/src/sgml/release-9.1.sgml +++ b/doc/src/sgml/release-9.1.sgml @@ -846,8 +846,7 @@ <listitem> <para> - Add functions to control streaming replication replay (Simon - Riggs) + Add functions to control streaming replication replay (Simon Riggs) </para> <para> @@ -1742,20 +1741,6 @@ </para> </listitem> - <listitem> - <para> - Minimize lock levels for <link - linkend="SQL-CREATETRIGGER"><command>CREATE TRIGGER</></link> - and many <link linkend="SQL-ALTERTABLE"><command>ALTER - TABLE</></link> and <link linkend="SQL-CREATERULE"><command>CREATE - RULE</></link> operations (Simon Riggs) - </para> - - <para> - This improves database availability when altering active databases. - </para> - </listitem> - </itemizedlist> </sect4> diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c index a3a99d28806..22864de957b 100644 --- a/src/backend/commands/tablecmds.c +++ b/src/backend/commands/tablecmds.c @@ -2578,6 +2578,31 @@ AlterTableInternal(Oid relid, List *cmds, bool recurse) LOCKMODE AlterTableGetLockLevel(List *cmds) { + /* + * Late in 9.1 dev cycle a number of issues were uncovered with access + * to catalog relations, leading to the decision to re-enforce all DDL + * at AccessExclusiveLock level by default. + * + * The issues are that there is a pervasive assumption in the code that + * the catalogs will not be read unless an AccessExclusiveLock is held. + * If that rule is relaxed, we must protect against a number of potential + * effects - infrequent, but proven possible with test cases where + * multiple DDL operations occur in a stream against frequently accessed + * tables. + * + * 1. Catalog tables are read using SnapshotNow, which has a race bug + * that allows a scan to return no valid rows even when one is present + * in the case of a commit of a concurrent update of the catalog table. + * SnapshotNow also ignores transactions in progress, so takes the + * latest committed version without waiting for the latest changes. + * + * 2. Relcache needs to be internally consistent, so unless we lock the + * definition during reads we have no way to guarantee that. + * + * 3. Catcache access isn't coordinated at all so refreshes can occur at + * any time. + */ +#ifdef REDUCED_ALTER_TABLE_LOCK_LEVELS ListCell *lcmd; LOCKMODE lockmode = ShareUpdateExclusiveLock; @@ -2726,6 +2751,9 @@ AlterTableGetLockLevel(List *cmds) if (cmd_lockmode > lockmode) lockmode = cmd_lockmode; } +#else + LOCKMODE lockmode = AccessExclusiveLock; +#endif return lockmode; } |