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Improve the manual's discussion of partitioning. Recommend using a
trigger instead of a rule to redirect insertions, use NEW.* notation where appropriate, some other updates and adjustments. David Fetter and Tom Lane
This commit is contained in:
Tom Lane
parent
8ee076325f
commit
43da837eda
@ -1,4 +1,4 @@
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<!-- $PostgreSQL: pgsql/doc/src/sgml/ddl.sgml,v 1.77 2007/11/28 15:42:31 petere Exp $ -->
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<!-- $PostgreSQL: pgsql/doc/src/sgml/ddl.sgml,v 1.78 2007/12/02 19:20:32 tgl Exp $ -->
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<chapter id="ddl">
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<title>Data Definition</title>
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@ -2383,8 +2383,8 @@ CHECK ( outletID BETWEEN 200 AND 300 )
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<listitem>
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<para>
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Optionally, define a rule or trigger to redirect modifications
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of the master table to the appropriate partition.
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Optionally, define a trigger or rule to redirect data inserted into
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the master table to the appropriate partition.
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</para>
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</listitem>
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@ -2443,16 +2443,16 @@ CREATE TABLE measurement (
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Next we create one partition for each active month:
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<programlisting>
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CREATE TABLE measurement_y2004m02 ( ) INHERITS (measurement);
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CREATE TABLE measurement_y2004m03 ( ) INHERITS (measurement);
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CREATE TABLE measurement_y2006m02 ( ) INHERITS (measurement);
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CREATE TABLE measurement_y2006m03 ( ) INHERITS (measurement);
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...
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CREATE TABLE measurement_y2005m11 ( ) INHERITS (measurement);
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CREATE TABLE measurement_y2005m12 ( ) INHERITS (measurement);
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CREATE TABLE measurement_y2006m01 ( ) INHERITS (measurement);
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CREATE TABLE measurement_y2007m11 ( ) INHERITS (measurement);
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CREATE TABLE measurement_y2007m12 ( ) INHERITS (measurement);
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CREATE TABLE measurement_y2008m01 ( ) INHERITS (measurement);
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</programlisting>
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Each of the partitions are complete tables in their own right,
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but they inherit their definition from the
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but they inherit their definitions from the
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<structname>measurement</> table.
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</para>
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@ -2470,21 +2470,21 @@ CREATE TABLE measurement_y2006m01 ( ) INHERITS (measurement);
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table creation script becomes:
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<programlisting>
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CREATE TABLE measurement_y2004m02 (
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CHECK ( logdate >= DATE '2004-02-01' AND logdate < DATE '2004-03-01' )
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CREATE TABLE measurement_y2006m02 (
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CHECK ( logdate >= DATE '2006-02-01' AND logdate < DATE '2006-03-01' )
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) INHERITS (measurement);
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CREATE TABLE measurement_y2004m03 (
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CHECK ( logdate >= DATE '2004-03-01' AND logdate < DATE '2004-04-01' )
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CREATE TABLE measurement_y2006m03 (
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CHECK ( logdate >= DATE '2006-03-01' AND logdate < DATE '2006-04-01' )
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) INHERITS (measurement);
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...
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CREATE TABLE measurement_y2005m11 (
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CHECK ( logdate >= DATE '2005-11-01' AND logdate < DATE '2005-12-01' )
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CREATE TABLE measurement_y2007m11 (
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CHECK ( logdate >= DATE '2007-11-01' AND logdate < DATE '2007-12-01' )
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) INHERITS (measurement);
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CREATE TABLE measurement_y2005m12 (
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CHECK ( logdate >= DATE '2005-12-01' AND logdate < DATE '2006-01-01' )
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CREATE TABLE measurement_y2007m12 (
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CHECK ( logdate >= DATE '2007-12-01' AND logdate < DATE '2008-01-01' )
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) INHERITS (measurement);
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CREATE TABLE measurement_y2006m01 (
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CHECK ( logdate >= DATE '2006-01-01' AND logdate < DATE '2006-02-01' )
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CREATE TABLE measurement_y2008m01 (
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CHECK ( logdate >= DATE '2008-01-01' AND logdate < DATE '2008-02-01' )
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) INHERITS (measurement);
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</programlisting>
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</para>
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@ -2495,12 +2495,12 @@ CREATE TABLE measurement_y2006m01 (
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We probably need indexes on the key columns too:
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<programlisting>
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CREATE INDEX measurement_y2004m02_logdate ON measurement_y2004m02 (logdate);
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CREATE INDEX measurement_y2004m03_logdate ON measurement_y2004m03 (logdate);
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CREATE INDEX measurement_y2006m02_logdate ON measurement_y2006m02 (logdate);
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CREATE INDEX measurement_y2006m03_logdate ON measurement_y2006m03 (logdate);
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...
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CREATE INDEX measurement_y2005m11_logdate ON measurement_y2005m11 (logdate);
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CREATE INDEX measurement_y2005m12_logdate ON measurement_y2005m12 (logdate);
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CREATE INDEX measurement_y2006m01_logdate ON measurement_y2006m01 (logdate);
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CREATE INDEX measurement_y2007m11_logdate ON measurement_y2007m11 (logdate);
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CREATE INDEX measurement_y2007m12_logdate ON measurement_y2007m12 (logdate);
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CREATE INDEX measurement_y2008m01_logdate ON measurement_y2008m01 (logdate);
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</programlisting>
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We choose not to add further indexes at this time.
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@ -2509,56 +2509,72 @@ CREATE INDEX measurement_y2006m01_logdate ON measurement_y2006m01 (logdate);
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<listitem>
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<para>
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We want our application to be able to say <literal>INSERT INTO
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measurement ...</> and have the data be redirected into the
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appropriate partition table. We can arrange that by attaching
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a suitable trigger function to the master table.
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If data will be added only to the latest partition, we can
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set up a very simple rule to insert data. We must
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redefine this each month so that it always points to the
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current partition:
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use a very simple trigger function:
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<programlisting>
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CREATE OR REPLACE RULE measurement_current_partition AS
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ON INSERT TO measurement
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DO INSTEAD
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INSERT INTO measurement_y2006m01 VALUES ( NEW.city_id,
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NEW.logdate,
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NEW.peaktemp,
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NEW.unitsales );
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CREATE OR REPLACE FUNCTION measurement_insert_trigger()
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RETURNS TRIGGER AS $$
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BEGIN
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INSERT INTO measurement_y2008m01 VALUES (NEW.*);
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RETURN NULL;
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END;
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$$
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LANGUAGE plpgsql;
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</programlisting>
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After creating the function, we create a trigger which
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calls the trigger function:
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<programlisting>
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CREATE TRIGGER insert_measurement_trigger
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BEFORE INSERT ON measurement
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FOR EACH ROW EXECUTE PROCEDURE measurement_insert_trigger();
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</programlisting>
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We must redefine the trigger function each month so that it always
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points to the current partition. The trigger definition does
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not need to be updated, however.
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</para>
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<para>
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We might want to insert data and have the server automatically
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locate the partition into which the row should be added. We
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could do this with a more complex set of rules as shown below:
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could do this with a more complex trigger function, for example:
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<programlisting>
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CREATE RULE measurement_insert_y2004m02 AS
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ON INSERT TO measurement WHERE
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( logdate >= DATE '2004-02-01' AND logdate < DATE '2004-03-01' )
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DO INSTEAD
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INSERT INTO measurement_y2004m02 VALUES ( NEW.city_id,
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NEW.logdate,
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NEW.peaktemp,
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NEW.unitsales );
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...
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CREATE RULE measurement_insert_y2005m12 AS
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ON INSERT TO measurement WHERE
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( logdate >= DATE '2005-12-01' AND logdate < DATE '2006-01-01' )
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DO INSTEAD
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INSERT INTO measurement_y2005m12 VALUES ( NEW.city_id,
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NEW.logdate,
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NEW.peaktemp,
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NEW.unitsales );
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CREATE RULE measurement_insert_y2006m01 AS
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ON INSERT TO measurement WHERE
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( logdate >= DATE '2006-01-01' AND logdate < DATE '2006-02-01' )
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DO INSTEAD
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INSERT INTO measurement_y2006m01 VALUES ( NEW.city_id,
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NEW.logdate,
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NEW.peaktemp,
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NEW.unitsales );
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CREATE OR REPLACE FUNCTION measurement_insert_trigger()
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RETURNS TRIGGER AS $$
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BEGIN
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IF ( logdate >= DATE '2006-02-01' AND logdate < DATE '2006-03-01' ) THEN
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INSERT INTO measurement_y2006m02 VALUES (NEW.*);
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ELSIF ( logdate >= DATE '2006-03-01' AND logdate < DATE '2006-04-01' ) THEN
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INSERT INTO measurement_y2006m03 VALUES (NEW.*);
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...
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ELSIF ( logdate >= DATE '2008-01-01' AND logdate < DATE '2008-02-01' ) THEN
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INSERT INTO measurement_y2008m01 VALUES (NEW.*);
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ELSE
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RAISE EXCEPTION 'Date out of range. Fix the measurement_insert_trigger() function!';
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END IF;
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RETURN NULL;
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END;
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$$
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LANGUAGE plpgsql;
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</programlisting>
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Note that the <literal>WHERE</literal> clause in each rule
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exactly matches the <literal>CHECK</literal>
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constraint for its partition.
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The trigger definition is the same as before.
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Note that each <literal>IF</literal> test must exactly match the
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<literal>CHECK</literal> constraint for its partition.
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</para>
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<para>
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While this function is more complex than the single-month case,
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it doesn't need to be updated as often, since branches can be
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added in advance of being needed.
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</para>
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</listitem>
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</orderedlist>
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@ -2571,24 +2587,6 @@ DO INSTEAD
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script that generates the required DDL automatically.
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</para>
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<para>
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Partitioning can also be arranged using a <literal>UNION ALL</literal>
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view:
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<programlisting>
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CREATE VIEW measurement AS
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SELECT * FROM measurement_y2004m02
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UNION ALL SELECT * FROM measurement_y2004m03
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...
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UNION ALL SELECT * FROM measurement_y2005m11
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UNION ALL SELECT * FROM measurement_y2005m12
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UNION ALL SELECT * FROM measurement_y2006m01;
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</programlisting>
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However, the need to
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recreate the view adds an extra step to adding and dropping
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individual partitions of the data set.
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</para>
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</sect2>
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<sect2 id="ddl-partitioning-managing-partitions">
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@ -2609,7 +2607,7 @@ UNION ALL SELECT * FROM measurement_y2006m01;
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The simplest option for removing old data is simply to drop the partition
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that is no longer necessary:
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<programlisting>
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DROP TABLE measurement_y2003m02;
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DROP TABLE measurement_y2006m02;
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</programlisting>
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This can very quickly delete millions of records because it doesn't have
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to individually delete every record.
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@ -2620,7 +2618,7 @@ DROP TABLE measurement_y2003m02;
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the partitioned table but retain access to it as a table in its own
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right:
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<programlisting>
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ALTER TABLE measurement_y2003m02 NO INHERIT measurement;
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ALTER TABLE measurement_y2006m02 NO INHERIT measurement;
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</programlisting>
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This allows further operations to be performed on the data before
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it is dropped. For example, this is often a useful time to back up
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@ -2636,8 +2634,8 @@ ALTER TABLE measurement_y2003m02 NO INHERIT measurement;
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were created above:
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<programlisting>
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CREATE TABLE measurement_y2006m02 (
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CHECK ( logdate >= DATE '2006-02-01' AND logdate < DATE '2006-03-01' )
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CREATE TABLE measurement_y2008m02 (
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CHECK ( logdate >= DATE '2008-02-01' AND logdate < DATE '2008-03-01' )
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) INHERITS (measurement);
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</programlisting>
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@ -2647,13 +2645,13 @@ CREATE TABLE measurement_y2006m02 (
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transformed prior to it appearing in the partitioned table:
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<programlisting>
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CREATE TABLE measurement_y2006m02
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CREATE TABLE measurement_y2008m02
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(LIKE measurement INCLUDING DEFAULTS INCLUDING CONSTRAINTS);
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ALTER TABLE measurement_y2006m02 ADD CONSTRAINT y2006m02
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CHECK ( logdate >= DATE '2006-02-01' AND logdate < DATE '2006-03-01' );
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\copy measurement_y2006m02 from 'measurement_y2006m02'
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ALTER TABLE measurement_y2008m02 ADD CONSTRAINT y2008m02
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CHECK ( logdate >= DATE '2008-02-01' AND logdate < DATE '2008-03-01' );
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\copy measurement_y2008m02 from 'measurement_y2008m02'
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-- possibly some other data preparation work
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ALTER TABLE measurement_y2006m02 INHERIT measurement;
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ALTER TABLE measurement_y2008m02 INHERIT measurement;
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</programlisting>
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</para>
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</sect2>
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@ -2672,7 +2670,7 @@ ALTER TABLE measurement_y2006m02 INHERIT measurement;
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<programlisting>
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SET constraint_exclusion = on;
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SELECT count(*) FROM measurement WHERE logdate >= DATE '2006-01-01';
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SELECT count(*) FROM measurement WHERE logdate >= DATE '2008-01-01';
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</programlisting>
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Without constraint exclusion, the above query would scan each of
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@ -2691,23 +2689,23 @@ SELECT count(*) FROM measurement WHERE logdate >= DATE '2006-01-01';
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<programlisting>
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SET constraint_exclusion = off;
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EXPLAIN SELECT count(*) FROM measurement WHERE logdate >= DATE '2006-01-01';
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EXPLAIN SELECT count(*) FROM measurement WHERE logdate >= DATE '2008-01-01';
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QUERY PLAN
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-----------------------------------------------------------------------------------------------
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Aggregate (cost=158.66..158.68 rows=1 width=0)
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-> Append (cost=0.00..151.88 rows=2715 width=0)
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-> Seq Scan on measurement (cost=0.00..30.38 rows=543 width=0)
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Filter: (logdate >= '2006-01-01'::date)
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-> Seq Scan on measurement_y2004m02 measurement (cost=0.00..30.38 rows=543 width=0)
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Filter: (logdate >= '2006-01-01'::date)
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-> Seq Scan on measurement_y2004m03 measurement (cost=0.00..30.38 rows=543 width=0)
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Filter: (logdate >= '2006-01-01'::date)
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Filter: (logdate >= '2008-01-01'::date)
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-> Seq Scan on measurement_y2006m02 measurement (cost=0.00..30.38 rows=543 width=0)
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Filter: (logdate >= '2008-01-01'::date)
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-> Seq Scan on measurement_y2006m03 measurement (cost=0.00..30.38 rows=543 width=0)
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Filter: (logdate >= '2008-01-01'::date)
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...
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-> Seq Scan on measurement_y2005m12 measurement (cost=0.00..30.38 rows=543 width=0)
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Filter: (logdate >= '2006-01-01'::date)
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-> Seq Scan on measurement_y2006m01 measurement (cost=0.00..30.38 rows=543 width=0)
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Filter: (logdate >= '2006-01-01'::date)
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-> Seq Scan on measurement_y2007m12 measurement (cost=0.00..30.38 rows=543 width=0)
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Filter: (logdate >= '2008-01-01'::date)
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-> Seq Scan on measurement_y2008m01 measurement (cost=0.00..30.38 rows=543 width=0)
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Filter: (logdate >= '2008-01-01'::date)
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</programlisting>
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Some or all of the partitions might use index scans instead of
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@ -2718,15 +2716,15 @@ EXPLAIN SELECT count(*) FROM measurement WHERE logdate >= DATE '2006-01-01';
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<programlisting>
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SET constraint_exclusion = on;
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EXPLAIN SELECT count(*) FROM measurement WHERE logdate >= DATE '2006-01-01';
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EXPLAIN SELECT count(*) FROM measurement WHERE logdate >= DATE '2008-01-01';
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QUERY PLAN
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-----------------------------------------------------------------------------------------------
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Aggregate (cost=63.47..63.48 rows=1 width=0)
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-> Append (cost=0.00..60.75 rows=1086 width=0)
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-> Seq Scan on measurement (cost=0.00..30.38 rows=543 width=0)
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Filter: (logdate >= '2006-01-01'::date)
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-> Seq Scan on measurement_y2006m01 measurement (cost=0.00..30.38 rows=543 width=0)
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Filter: (logdate >= '2006-01-01'::date)
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Filter: (logdate >= '2008-01-01'::date)
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-> Seq Scan on measurement_y2008m01 measurement (cost=0.00..30.38 rows=543 width=0)
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Filter: (logdate >= '2008-01-01'::date)
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</programlisting>
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</para>
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@ -2739,6 +2737,69 @@ EXPLAIN SELECT count(*) FROM measurement WHERE logdate >= DATE '2006-01-01';
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a large part of the partition or just a small part. An index will
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be helpful in the latter case but not the former.
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</para>
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</sect2>
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<sect2 id="ddl-partitioning-alternatives">
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<title>Alternative Partitioning Methods</title>
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<para>
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A different approach to redirecting inserts into the appropriate
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partition table is to set up rules, instead of a trigger, on the
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master table. For example:
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<programlisting>
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CREATE RULE measurement_insert_y2006m02 AS
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ON INSERT TO measurement WHERE
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( logdate >= DATE '2006-02-01' AND logdate < DATE '2006-03-01' )
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DO INSTEAD
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INSERT INTO measurement_y2006m02 VALUES (NEW.*);
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...
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CREATE RULE measurement_insert_y2008m01 AS
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ON INSERT TO measurement WHERE
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( logdate >= DATE '2008-01-01' AND logdate < DATE '2008-02-01' )
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DO INSTEAD
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INSERT INTO measurement_y2008m01 VALUES (NEW.*);
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</programlisting>
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A rule has significantly more overhead than a trigger, but the overhead
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is paid once per query rather than once per row, so this method might be
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advantageous for bulk-insert situations. In most cases, however, the
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trigger method will offer better performance.
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</para>
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<para>
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Be aware that <command>COPY</> ignores rules. If you want to
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use <command>COPY</> to insert data, you'll need to copy into the correct
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partition table rather than into the master. <command>COPY</> does fire
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triggers, so you can use it normally if you use the trigger approach.
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</para>
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<para>
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Another disadvantage of the rule approach is that there is no simple
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way to force an error if the set of rules doesn't cover the insertion
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date; the data will silently go into the master table instead.
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</para>
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<para>
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Partitioning can also be arranged using a <literal>UNION ALL</literal>
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view, instead of table inheritance. For example,
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|
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<programlisting>
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CREATE VIEW measurement AS
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SELECT * FROM measurement_y2006m02
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UNION ALL SELECT * FROM measurement_y2006m03
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...
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UNION ALL SELECT * FROM measurement_y2007m11
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UNION ALL SELECT * FROM measurement_y2007m12
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UNION ALL SELECT * FROM measurement_y2008m01;
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</programlisting>
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However, the need to recreate the view adds an extra step to adding and
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dropping individual partitions of the data set. In practice this
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method has little to recommend it compared to using inheritance.
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</para>
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</sect2>
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<sect2 id="ddl-partitioning-caveats">
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@ -2749,24 +2810,38 @@ EXPLAIN SELECT count(*) FROM measurement WHERE logdate >= DATE '2006-01-01';
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<itemizedlist>
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<listitem>
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<para>
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There is currently no way to verify that all of the
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There is no automatic way to verify that all of the
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<literal>CHECK</literal> constraints are mutually
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||||
exclusive. Care is required by the database designer.
|
||||
exclusive. It is safer to create code that generates
|
||||
partitions and creates and/or modifies associated objects than
|
||||
to write each by hand.
|
||||
</para>
|
||||
</listitem>
|
||||
|
||||
<listitem>
|
||||
<para>
|
||||
There is currently no simple way to specify that rows must not be
|
||||
inserted into the master table. A <literal>CHECK (false)</literal>
|
||||
constraint on the master table would be inherited by all child
|
||||
tables, so that cannot be used for this purpose. One possibility is
|
||||
to set up an <literal>ON INSERT</> trigger on the master table that
|
||||
always raises an error. (Alternatively, such a trigger could be
|
||||
used to redirect the data into the proper child table, instead of
|
||||
using a set of rules as suggested above.)
|
||||
The schemes shown here assume that the partition key column(s)
|
||||
of a row never change, or at least do not change enough to require
|
||||
it to move to another partition. An <command>UPDATE</> that attempts
|
||||
to do that will fail because of the <literal>CHECK</> constraints.
|
||||
If you need to handle such cases, you can put suitable update triggers
|
||||
on the partition tables, but it makes management of the structure
|
||||
much more complicated.
|
||||
</para>
|
||||
</listitem>
|
||||
|
||||
<listitem>
|
||||
<para>
|
||||
If you are using manual <command>VACUUM</command> or
|
||||
<command>ANALYZE</command> commands, don't forget that
|
||||
you need to run them on each partition individually. A command like
|
||||
<programlisting>
|
||||
ANALYZE measurement;
|
||||
</programlisting>
|
||||
will only process the master table.
|
||||
</para>
|
||||
</listitem>
|
||||
|
||||
</itemizedlist>
|
||||
</para>
|
||||
|
||||
@ -2801,18 +2876,9 @@ EXPLAIN SELECT count(*) FROM measurement WHERE logdate >= DATE '2006-01-01';
|
||||
<para>
|
||||
All constraints on all partitions of the master table are examined
|
||||
during constraint exclusion, so large numbers of partitions are likely
|
||||
to increase query planning time considerably.
|
||||
</para>
|
||||
</listitem>
|
||||
|
||||
<listitem>
|
||||
<para>
|
||||
Don't forget that you still need to run <command>ANALYZE</command>
|
||||
on each partition individually. A command like:
|
||||
<programlisting>
|
||||
ANALYZE measurement;
|
||||
</programlisting>
|
||||
will only process the master table.
|
||||
to increase query planning time considerably. Partitioning using
|
||||
these techniques will work well with up to perhaps a hundred partitions;
|
||||
don't try to use many thousands of partitions.
|
||||
</para>
|
||||
</listitem>
|
||||
|
||||
|
||||
Loading…
Reference in New Issue
Block a user