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Commit2c3d9db

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Reset ALTER TABLE lock levels to AccessExclusiveLock in all cases.
Locks on inheritance parent remain at lower level, as they were before.Remove entry from 9.1 release notes.
1 parent4fa046a commit2c3d9db

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2 files changed

+29
-16
lines changed

2 files changed

+29
-16
lines changed

‎doc/src/sgml/release-9.1.sgml

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@@ -846,8 +846,7 @@
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<listitem>
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<para>
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Add functions to control streaming replication replay (Simon
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Riggs)
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Add functions to control streaming replication replay (Simon Riggs)
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</para>
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<para>
@@ -1742,20 +1741,6 @@
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</para>
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</listitem>
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<listitem>
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<para>
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Minimize lock levels for <link
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linkend="SQL-CREATETRIGGER"><command>CREATE TRIGGER</></link>
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and many <link linkend="SQL-ALTERTABLE"><command>ALTER
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TABLE</></link> and <link linkend="SQL-CREATERULE"><command>CREATE
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RULE</></link> operations (Simon Riggs)
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</para>
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<para>
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This improves database availability when altering active databases.
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</para>
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</listitem>
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</itemizedlist>
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</sect4>

‎src/backend/commands/tablecmds.c

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Original file line numberDiff line numberDiff line change
@@ -2578,6 +2578,31 @@ AlterTableInternal(Oid relid, List *cmds, bool recurse)
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LOCKMODE
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AlterTableGetLockLevel(List*cmds)
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{
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/*
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* Late in 9.1 dev cycle a number of issues were uncovered with access
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* to catalog relations, leading to the decision to re-enforce all DDL
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* at AccessExclusiveLock level by default.
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*
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* The issues are that there is a pervasive assumption in the code that
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* the catalogs will not be read unless an AccessExclusiveLock is held.
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* If that rule is relaxed, we must protect against a number of potential
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* effects - infrequent, but proven possible with test cases where
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* multiple DDL operations occur in a stream against frequently accessed
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* tables.
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*
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* 1. Catalog tables are read using SnapshotNow, which has a race bug
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* that allows a scan to return no valid rows even when one is present
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* in the case of a commit of a concurrent update of the catalog table.
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* SnapshotNow also ignores transactions in progress, so takes the
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* latest committed version without waiting for the latest changes.
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*
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* 2. Relcache needs to be internally consistent, so unless we lock the
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* definition during reads we have no way to guarantee that.
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*
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* 3. Catcache access isn't coordinated at all so refreshes can occur at
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* any time.
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*/
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#ifdefREDUCED_ALTER_TABLE_LOCK_LEVELS
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ListCell*lcmd;
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LOCKMODElockmode=ShareUpdateExclusiveLock;
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@@ -2726,6 +2751,9 @@ AlterTableGetLockLevel(List *cmds)
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if (cmd_lockmode>lockmode)
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lockmode=cmd_lockmode;
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}
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#else
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LOCKMODElockmode=AccessExclusiveLock;
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#endif
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returnlockmode;
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}

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