-<!-- $PostgreSQL: pgsql/doc/src/sgml/mvcc.sgml,v 2.60 2006/09/16 00:30:14 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/mvcc.sgml,v 2.61 2006/09/17 22:50:31 tgl Exp $ -->
<chapter id="mvcc">
<title>Concurrency Control</title>
</term>
<listitem>
<para>
- A transaction reads data written by a concurrent uncommitted transaction.
+ A transaction reads data written by a concurrent uncommitted transaction.
</para>
</listitem>
</varlistentry>
</term>
<listitem>
<para>
- A transaction re-reads data it has previously read and finds that data
- has been modified by another transaction (that committed since the
- initial read).
+ A transaction re-reads data it has previously read and finds that data
+ has been modified by another transaction (that committed since the
+ initial read).
</para>
</listitem>
</varlistentry>
</term>
<listitem>
<para>
- A transaction re-executes a query returning a set of rows that satisfy a
- search condition and finds that the set of rows satisfying the condition
- has changed due to another recently-committed transaction.
+ A transaction re-executes a query returning a set of rows that satisfy a
+ search condition and finds that the set of rows satisfying the condition
+ has changed due to another recently-committed transaction.
</para>
</listitem>
</varlistentry>
<tgroup cols="4">
<thead>
<row>
- <entry>
+ <entry>
Isolation Level
- </entry>
- <entry>
- Dirty Read
- </entry>
- <entry>
- Nonrepeatable Read
- </entry>
- <entry>
- Phantom Read
- </entry>
+ </entry>
+ <entry>
+ Dirty Read
+ </entry>
+ <entry>
+ Nonrepeatable Read
+ </entry>
+ <entry>
+ Phantom Read
+ </entry>
</row>
</thead>
<tbody>
<row>
- <entry>
- Read uncommitted
- </entry>
- <entry>
- Possible
- </entry>
- <entry>
- Possible
- </entry>
- <entry>
- Possible
- </entry>
+ <entry>
+ Read uncommitted
+ </entry>
+ <entry>
+ Possible
+ </entry>
+ <entry>
+ Possible
+ </entry>
+ <entry>
+ Possible
+ </entry>
</row>
<row>
- <entry>
- Read committed
- </entry>
- <entry>
- Not possible
- </entry>
- <entry>
- Possible
- </entry>
- <entry>
- Possible
- </entry>
+ <entry>
+ Read committed
+ </entry>
+ <entry>
+ Not possible
+ </entry>
+ <entry>
+ Possible
+ </entry>
+ <entry>
+ Possible
+ </entry>
</row>
<row>
- <entry>
- Repeatable read
- </entry>
- <entry>
- Not possible
- </entry>
- <entry>
- Not possible
- </entry>
- <entry>
- Possible
- </entry>
+ <entry>
+ Repeatable read
+ </entry>
+ <entry>
+ Not possible
+ </entry>
+ <entry>
+ Not possible
+ </entry>
+ <entry>
+ Possible
+ </entry>
</row>
<row>
- <entry>
- Serializable
- </entry>
- <entry>
- Not possible
- </entry>
- <entry>
- Not possible
- </entry>
- <entry>
- Not possible
- </entry>
+ <entry>
+ Serializable
+ </entry>
+ <entry>
+ Not possible
+ </entry>
+ <entry>
+ Not possible
+ </entry>
+ <entry>
+ Not possible
+ </entry>
</row>
</tbody>
</tgroup>
<title>Table-level lock modes</title>
<varlistentry>
<term>
- <literal>ACCESS SHARE</literal>
+ <literal>ACCESS SHARE</literal>
</term>
<listitem>
- <para>
- Conflicts with the <literal>ACCESS EXCLUSIVE</literal> lock
- mode only.
- </para>
-
- <para>
- The commands <command>SELECT</command> and
- <command>ANALYZE</command> acquire a lock of this mode on
- referenced tables. In general, any query that only reads a table
- and does not modify it will acquire this lock mode.
- </para>
+ <para>
+ Conflicts with the <literal>ACCESS EXCLUSIVE</literal> lock
+ mode only.
+ </para>
+
+ <para>
+ The <command>SELECT</command> command acquires a lock of this mode on
+ referenced tables. In general, any query that only reads a table
+ and does not modify it will acquire this lock mode.
+ </para>
</listitem>
</varlistentry>
<varlistentry>
<term>
- <literal>ROW SHARE</literal>
+ <literal>ROW SHARE</literal>
</term>
<listitem>
- <para>
- Conflicts with the <literal>EXCLUSIVE</literal> and
- <literal>ACCESS EXCLUSIVE</literal> lock modes.
- </para>
-
- <para>
- The <command>SELECT FOR UPDATE</command> and
- <command>SELECT FOR SHARE</command> commands acquire a
- lock of this mode on the target table(s) (in addition to
- <literal>ACCESS SHARE</literal> locks on any other tables
- that are referenced but not selected
- <option>FOR UPDATE/FOR SHARE</option>).
- </para>
+ Conflicts with the <literal>EXCLUSIVE</literal> and
+ <literal>ACCESS EXCLUSIVE</literal> lock modes.
+ </para>
+
+ The <command>SELECT FOR UPDATE</command> and
+ <command>SELECT FOR SHARE</command> commands acquire a
+ lock of this mode on the target table(s) (in addition to
+ <literal>ACCESS SHARE</literal> locks on any other tables
+ that are referenced but not selected
+ <option>FOR UPDATE/FOR SHARE</option>).
+ </para>
</listitem>
</varlistentry>
<varlistentry>
<term>
- <literal>ROW EXCLUSIVE</literal>
+ <literal>ROW EXCLUSIVE</literal>
</term>
<listitem>
- <para>
- Conflicts with the <literal>SHARE</literal>, <literal>SHARE ROW
- EXCLUSIVE</literal>, <literal>EXCLUSIVE</literal>, and
- <literal>ACCESS EXCLUSIVE</literal> lock modes.
- </para>
-
- <para>
- The commands <command>UPDATE</command>,
- <command>DELETE</command>, and <command>INSERT</command>
- acquire this lock mode on the target table (in addition to
- <literal>ACCESS SHARE</literal> locks on any other referenced
- tables). In general, this lock mode will be acquired by any
- command that modifies the data in a table.
- </para>
+ Conflicts with the <literal>SHARE</literal>, <literal>SHARE ROW
+ EXCLUSIVE</literal>, <literal>EXCLUSIVE</literal>, and
+ <literal>ACCESS EXCLUSIVE</literal> lock modes.
+ </para>
+
+ The commands <command>UPDATE</command>,
+ <command>DELETE</command>, and <command>INSERT</command>
+ acquire this lock mode on the target table (in addition to
+ <literal>ACCESS SHARE</literal> locks on any other referenced
+ tables). In general, this lock mode will be acquired by any
+ command that modifies the data in a table.
+ </para>
</listitem>
</varlistentry>
<varlistentry>
<term>
- <literal>SHARE UPDATE EXCLUSIVE</literal>
+ <literal>SHARE UPDATE EXCLUSIVE</literal>
</term>
<listitem>
- <para>
- Conflicts with the <literal>SHARE UPDATE EXCLUSIVE</literal>,
- <literal>SHARE</literal>, <literal>SHARE ROW
- EXCLUSIVE</literal>, <literal>EXCLUSIVE</literal>, and
- <literal>ACCESS EXCLUSIVE</literal> lock modes.
- This mode protects a table against
- concurrent schema changes and <command>VACUUM</> runs.
- </para>
-
- <para>
- Acquired by <command>VACUUM</command> (without <option>FULL</option>)
- and by <command>CREATE INDEX CONCURRENTLY</>.
- </para>
+ Conflicts with the <literal>SHARE UPDATE EXCLUSIVE</literal>,
+ <literal>SHARE</literal>, <literal>SHARE ROW
+ EXCLUSIVE</literal>, <literal>EXCLUSIVE</literal>, and
+ <literal>ACCESS EXCLUSIVE</literal> lock modes.
+ This mode protects a table against
+ concurrent schema changes and <command>VACUUM</> runs.
+ </para>
+
+ Acquired by <command>VACUUM</command> (without <option>FULL</option>),
+ <command>ANALYZE</>, and <command>CREATE INDEX CONCURRENTLY</>.
+ </para>
</listitem>
</varlistentry>
<varlistentry>
<term>
- <literal>SHARE</literal>
+ <literal>SHARE</literal>
</term>
<listitem>
- <para>
- Conflicts with the <literal>ROW EXCLUSIVE</literal>,
- <literal>SHARE UPDATE EXCLUSIVE</literal>, <literal>SHARE ROW
- EXCLUSIVE</literal>, <literal>EXCLUSIVE</literal>, and
- <literal>ACCESS EXCLUSIVE</literal> lock modes.
- This mode protects a table against concurrent data changes.
- </para>
-
- <para>
- Acquired by <command>CREATE INDEX</command>
- (without <option>CONCURRENTLY</option>).
- </para>
+ Conflicts with the <literal>ROW EXCLUSIVE</literal>,
+ <literal>SHARE UPDATE EXCLUSIVE</literal>, <literal>SHARE ROW
+ EXCLUSIVE</literal>, <literal>EXCLUSIVE</literal>, and
+ <literal>ACCESS EXCLUSIVE</literal> lock modes.
+ This mode protects a table against concurrent data changes.
+ </para>
+
+ Acquired by <command>CREATE INDEX</command>
+ (without <option>CONCURRENTLY</option>).
+ </para>
</listitem>
</varlistentry>
<varlistentry>
<term>
- <literal>SHARE ROW EXCLUSIVE</literal>
+ <literal>SHARE ROW EXCLUSIVE</literal>
</term>
<listitem>
- <para>
- Conflicts with the <literal>ROW EXCLUSIVE</literal>,
- <literal>SHARE UPDATE EXCLUSIVE</literal>,
- <literal>SHARE</literal>, <literal>SHARE ROW
- EXCLUSIVE</literal>, <literal>EXCLUSIVE</literal>, and
- <literal>ACCESS EXCLUSIVE</literal> lock modes.
- </para>
-
- <para>
+ Conflicts with the <literal>ROW EXCLUSIVE</literal>,
+ <literal>SHARE UPDATE EXCLUSIVE</literal>,
+ <literal>SHARE</literal>, <literal>SHARE ROW
+ EXCLUSIVE</literal>, <literal>EXCLUSIVE</literal>, and
+ <literal>ACCESS EXCLUSIVE</literal> lock modes.
+ </para>
+
This lock mode is not automatically acquired by any
<productname>PostgreSQL</productname> command.
- </para>
+ </para>
</listitem>
</varlistentry>
<varlistentry>
<term>
- <literal>EXCLUSIVE</literal>
+ <literal>EXCLUSIVE</literal>
</term>
<listitem>
- <para>
- Conflicts with the <literal>ROW SHARE</literal>, <literal>ROW
- EXCLUSIVE</literal>, <literal>SHARE UPDATE
- EXCLUSIVE</literal>, <literal>SHARE</literal>, <literal>SHARE
- ROW EXCLUSIVE</literal>, <literal>EXCLUSIVE</literal>, and
- <literal>ACCESS EXCLUSIVE</literal> lock modes.
- This mode allows only concurrent <literal>ACCESS SHARE</literal> locks,
- i.e., only reads from the table can proceed in parallel with a
- transaction holding this lock mode.
- </para>
-
- <para>
+ Conflicts with the <literal>ROW SHARE</literal>, <literal>ROW
+ EXCLUSIVE</literal>, <literal>SHARE UPDATE
+ EXCLUSIVE</literal>, <literal>SHARE</literal>, <literal>SHARE
+ ROW EXCLUSIVE</literal>, <literal>EXCLUSIVE</literal>, and
+ <literal>ACCESS EXCLUSIVE</literal> lock modes.
+ This mode allows only concurrent <literal>ACCESS SHARE</literal> locks,
+ i.e., only reads from the table can proceed in parallel with a
+ transaction holding this lock mode.
+ </para>
+
This lock mode is not automatically acquired on user tables by any
<productname>PostgreSQL</productname> command. However it is
acquired on certain system catalogs in some operations.
- </para>
+ </para>
</listitem>
</varlistentry>
<varlistentry>
<term>
- <literal>ACCESS EXCLUSIVE</literal>
+ <literal>ACCESS EXCLUSIVE</literal>
</term>
<listitem>
- <para>
- Conflicts with locks of all modes (<literal>ACCESS
- SHARE</literal>, <literal>ROW SHARE</literal>, <literal>ROW
- EXCLUSIVE</literal>, <literal>SHARE UPDATE
- EXCLUSIVE</literal>, <literal>SHARE</literal>, <literal>SHARE
- ROW EXCLUSIVE</literal>, <literal>EXCLUSIVE</literal>, and
- <literal>ACCESS EXCLUSIVE</literal>).
- This mode guarantees that the
- holder is the only transaction accessing the table in any way.
- </para>
-
- <para>
- Acquired by the <command>ALTER TABLE</command>, <command>DROP
- TABLE</command>, <command>TRUNCATE</command>, <command>REINDEX</command>,
- <command>CLUSTER</command>, and <command>VACUUM FULL</command>
- commands. This is also the default lock mode for <command>LOCK
- TABLE</command> statements that do not specify a mode explicitly.
- </para>
+ Conflicts with locks of all modes (<literal>ACCESS
+ SHARE</literal>, <literal>ROW SHARE</literal>, <literal>ROW
+ EXCLUSIVE</literal>, <literal>SHARE UPDATE
+ EXCLUSIVE</literal>, <literal>SHARE</literal>, <literal>SHARE
+ ROW EXCLUSIVE</literal>, <literal>EXCLUSIVE</literal>, and
+ <literal>ACCESS EXCLUSIVE</literal>).
+ This mode guarantees that the
+ holder is the only transaction accessing the table in any way.
+ </para>
+
+ Acquired by the <command>ALTER TABLE</command>, <command>DROP
+ TABLE</command>, <command>TRUNCATE</command>, <command>REINDEX</command>,
+ <command>CLUSTER</command>, and <command>VACUUM FULL</command>
+ commands. This is also the default lock mode for <command>LOCK
+ TABLE</command> statements that do not specify a mode explicitly.
+ </para>
</listitem>
</varlistentry>
</variablelist>
</term>
<listitem>
<para>
- Short-term share/exclusive page-level locks are used for
- read/write access. Locks are released immediately after each
- index row is fetched or inserted. However, note that GIN index
- usually requires several inserts per one table row.
+ Short-term share/exclusive page-level locks are used for
+ read/write access. Locks are released immediately after each
+ index row is fetched or inserted. However, note that a GIN index
+ usually requires several inserts for each table row.
</para>
</listitem>
</varlistentry>
*
*
* IDENTIFICATION
- * $PostgreSQL: pgsql/src/backend/commands/analyze.c,v 1.97 2006/08/18 16:09:08 tgl Exp $
+ * $PostgreSQL: pgsql/src/backend/commands/analyze.c,v 1.98 2006/09/17 22:50:31 tgl Exp $
*
*-------------------------------------------------------------------------
*/
CHECK_FOR_INTERRUPTS();
/*
- * Open the relation, getting only a read lock on it. If the rel has
- * been dropped since we last saw it, we don't need to process it.
+ * Open the relation, getting ShareUpdateExclusiveLock to ensure that
+ * two ANALYZEs don't run on it concurrently. (This also locks out
+ * a concurrent VACUUM, which doesn't matter much at the moment but
+ * might matter if we ever try to accumulate stats on dead tuples.)
+ * If the rel has been dropped since we last saw it, we don't need
+ * to process it.
*/
- onerel = try_relation_open(relid, AccessShareLock);
+ onerel = try_relation_open(relid, ShareUpdateExclusiveLock);
if (!onerel)
return;
ereport(WARNING,
(errmsg("skipping \"%s\" --- only table or database owner can analyze it",
RelationGetRelationName(onerel))));
- relation_close(onerel, AccessShareLock);
+ relation_close(onerel, ShareUpdateExclusiveLock);
return;
}
ereport(WARNING,
(errmsg("skipping \"%s\" --- cannot analyze indexes, views, or special system tables",
RelationGetRelationName(onerel))));
- relation_close(onerel, AccessShareLock);
+ relation_close(onerel, ShareUpdateExclusiveLock);
return;
}
*/
if (isOtherTempNamespace(RelationGetNamespace(onerel)))
{
- relation_close(onerel, AccessShareLock);
+ relation_close(onerel, ShareUpdateExclusiveLock);
return;
}
*/
if (RelationGetRelid(onerel) == StatisticRelationId)
{
- relation_close(onerel, AccessShareLock);
+ relation_close(onerel, ShareUpdateExclusiveLock);
return;
}
0, 0);
vac_close_indexes(nindexes, Irel, AccessShareLock);
- relation_close(onerel, AccessShareLock);
+ relation_close(onerel, ShareUpdateExclusiveLock);
return;
}
/*
* Close source relation now, but keep lock so that no one deletes it
* before we commit. (If someone did, they'd fail to clean up the entries
- * we made in pg_statistic.)
+ * we made in pg_statistic. Also, releasing the lock before commit would
+ * expose us to concurrent-update failures in update_attstats.)
*/
relation_close(onerel, NoLock);
}
* Note analyze_rel() has seen to it that we won't come here when
* vacuuming pg_statistic itself.
*
- * Note: if two backends concurrently try to analyze the same relation,
- * the second one is likely to fail here with a "tuple concurrently
- * updated" error. This is slightly annoying, but no real harm is done.
- * We could prevent the problem by using a stronger lock on the
- * relation for ANALYZE (ie, ShareUpdateExclusiveLock instead
- * of AccessShareLock); but that cure seems worse than the disease,
- * especially now that ANALYZE doesn't start a new transaction
- * for each relation. The lock could be held for a long time...
+ * Note: there would be a race condition here if two backends could
+ * ANALYZE the same table concurrently. Presently, we lock that out
+ * by taking a self-exclusive lock on the relation in analyze_rel().
*/
static void
update_attstats(Oid relid, int natts, VacAttrStats **vacattrstats)
else
{
/* No, insert new tuple */
- stup = heap_formtuple(sd->rd_att, values, nulls);
+ stup = heap_formtuple(RelationGetDescr(sd), values, nulls);
simple_heap_insert(sd, stup);
}
projects / postgresql / commitdiff