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CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="DDL-PARTITIONING"
>5.9. Partitioning</A
></H1
><P
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> supports basic table
partitioning. This section describes why and how to implement
partitioning as part of your database design.
</P
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="DDL-PARTITIONING-OVERVIEW"
>5.9.1. Overview</A
></H2
><P
> Partitioning refers to splitting what is logically one large table
into smaller physical pieces.
Partitioning can provide several benefits:
<P
></P
></P><UL
><LI
><P
> Query performance can be improved dramatically in certain situations,
particularly when most of the heavily accessed rows of the table are in a
single partition or a small number of partitions. The partitioning
substitutes for leading columns of indexes, reducing index size and
making it more likely that the heavily-used parts of the indexes
fit in memory.
</P
></LI
><LI
><P
> When queries or updates access a large percentage of a single
partition, performance can be improved by taking advantage
of sequential scan of that partition instead of using an
index and random access reads scattered across the whole table.
</P
></LI
><LI
><P
> Bulk loads and deletes can be accomplished by adding or removing
partitions, if that requirement is planned into the partitioning design.
<TT
CLASS="COMMAND"
>ALTER TABLE NO INHERIT</TT
> and <TT
CLASS="COMMAND"
>DROP TABLE</TT
> are
both far faster than a bulk operation.
These commands also entirely avoid the <TT
CLASS="COMMAND"
>VACUUM</TT
>
overhead caused by a bulk <TT
CLASS="COMMAND"
>DELETE</TT
>.
</P
></LI
><LI
><P
> Seldom-used data can be migrated to cheaper and slower storage media.
</P
></LI
></UL
><P>
The benefits will normally be worthwhile only when a table would
otherwise be very large. The exact point at which a table will
benefit from partitioning depends on the application, although a
rule of thumb is that the size of the table should exceed the physical
memory of the database server.
</P
><P
> Currently, <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> supports partitioning
via table inheritance. Each partition must be created as a child
table of a single parent table. The parent table itself is normally
empty; it exists just to represent the entire data set. You should be
familiar with inheritance (see <A
HREF="ddl-inherit.html"
>Section 5.8</A
>) before
attempting to set up partitioning.
</P
><P
> The following forms of partitioning can be implemented in
<SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>:
<P
></P
></P><DIV
CLASS="VARIABLELIST"
><DL
><DT
>Range Partitioning</DT
><DD
><P
> The table is partitioned into <SPAN
CLASS="QUOTE"
>"ranges"</SPAN
> defined
by a key column or set of columns, with no overlap between
the ranges of values assigned to different partitions. For
example one might partition by date ranges, or by ranges of
identifiers for particular business objects.
</P
></DD
><DT
>List Partitioning</DT
><DD
><P
> The table is partitioned by explicitly listing which key values
appear in each partition.
</P
></DD
></DL
></DIV
><P>
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="DDL-PARTITIONING-IMPLEMENTATION"
>5.9.2. Implementing Partitioning</A
></H2
><P
> To set up a partitioned table, do the following:
<P
></P
></P><OL
COMPACT="COMPACT"
TYPE="1"
><LI
><P
> Create the <SPAN
CLASS="QUOTE"
>"master"</SPAN
> table, from which all of the
partitions will inherit.
</P
><P
> This table will contain no data. Do not define any check
constraints on this table, unless you intend them to
be applied equally to all partitions. There is no point
in defining any indexes or unique constraints on it, either.
</P
></LI
><LI
><P
> Create several <SPAN
CLASS="QUOTE"
>"child"</SPAN
> tables that each inherit from
the master table. Normally, these tables will not add any columns
to the set inherited from the master.
</P
><P
> We will refer to the child tables as partitions, though they
are in every way normal <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> tables.
</P
></LI
><LI
><P
> Add table constraints to the partition tables to define the
allowed key values in each partition.
</P
><P
> Typical examples would be:
</P><PRE
CLASS="PROGRAMLISTING"
>CHECK ( x = 1 )
CHECK ( county IN ( 'Oxfordshire', 'Buckinghamshire', 'Warwickshire' ))
CHECK ( outletID >= 100 AND outletID < 200 )</PRE
><P>
Ensure that the constraints guarantee that there is no overlap
between the key values permitted in different partitions. A common
mistake is to set up range constraints like:
</P><PRE
CLASS="PROGRAMLISTING"
>CHECK ( outletID BETWEEN 100 AND 200 )
CHECK ( outletID BETWEEN 200 AND 300 )</PRE
><P>
This is wrong since it is not clear which partition the key value
200 belongs in.
</P
><P
> Note that there is no difference in
syntax between range and list partitioning; those terms are
descriptive only.
</P
></LI
><LI
><P
> For each partition, create an index on the key column(s),
as well as any other indexes you might want. (The key index is
not strictly necessary, but in most scenarios it is helpful.
If you intend the key values to be unique then you should
always create a unique or primary-key constraint for each
partition.)
</P
></LI
><LI
><P
> Optionally, define a trigger or rule to redirect data inserted into
the master table to the appropriate partition.
</P
></LI
><LI
><P
> Ensure that the <A
HREF="runtime-config-query.html#GUC-CONSTRAINT-EXCLUSION"
>constraint_exclusion</A
>
configuration parameter is not disabled in
<TT
CLASS="FILENAME"
>postgresql.conf</TT
>.
If it is, queries will not be optimized as desired.
</P
></LI
></OL
><P>
</P
><P
> For example, suppose we are constructing a database for a large
ice cream company. The company measures peak temperatures every
day as well as ice cream sales in each region. Conceptually,
we want a table like:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TABLE measurement (
city_id int not null,
logdate date not null,
peaktemp int,
unitsales int
);</PRE
><P>
We know that most queries will access just the last week's, month's or
quarter's data, since the main use of this table will be to prepare
online reports for management.
To reduce the amount of old data that needs to be stored, we
decide to only keep the most recent 3 years worth of data. At the
beginning of each month we will remove the oldest month's data.
</P
><P
> In this situation we can use partitioning to help us meet all of our
different requirements for the measurements table. Following the
steps outlined above, partitioning can be set up as follows:
</P
><P
> <P
></P
></P><OL
COMPACT="COMPACT"
TYPE="1"
><LI
><P
> The master table is the <TT
CLASS="STRUCTNAME"
>measurement</TT
> table, declared
exactly as above.
</P
></LI
><LI
><P
> Next we create one partition for each active month:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TABLE measurement_y2006m02 ( ) INHERITS (measurement);
CREATE TABLE measurement_y2006m03 ( ) INHERITS (measurement);
...
CREATE TABLE measurement_y2007m11 ( ) INHERITS (measurement);
CREATE TABLE measurement_y2007m12 ( ) INHERITS (measurement);
CREATE TABLE measurement_y2008m01 ( ) INHERITS (measurement);</PRE
><P>
Each of the partitions are complete tables in their own right,
but they inherit their definitions from the
<TT
CLASS="STRUCTNAME"
>measurement</TT
> table.
</P
><P
> This solves one of our problems: deleting old data. Each
month, all we will need to do is perform a <TT
CLASS="COMMAND"
>DROP
TABLE</TT
> on the oldest child table and create a new
child table for the new month's data.
</P
></LI
><LI
><P
> We must provide non-overlapping table constraints. Rather than
just creating the partition tables as above, the table creation
script should really be:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TABLE measurement_y2006m02 (
CHECK ( logdate >= DATE '2006-02-01' AND logdate < DATE '2006-03-01' )
) INHERITS (measurement);
CREATE TABLE measurement_y2006m03 (
CHECK ( logdate >= DATE '2006-03-01' AND logdate < DATE '2006-04-01' )
) INHERITS (measurement);
...
CREATE TABLE measurement_y2007m11 (
CHECK ( logdate >= DATE '2007-11-01' AND logdate < DATE '2007-12-01' )
) INHERITS (measurement);
CREATE TABLE measurement_y2007m12 (
CHECK ( logdate >= DATE '2007-12-01' AND logdate < DATE '2008-01-01' )
) INHERITS (measurement);
CREATE TABLE measurement_y2008m01 (
CHECK ( logdate >= DATE '2008-01-01' AND logdate < DATE '2008-02-01' )
) INHERITS (measurement);</PRE
><P>
</P
></LI
><LI
><P
> We probably need indexes on the key columns too:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE INDEX measurement_y2006m02_logdate ON measurement_y2006m02 (logdate);
CREATE INDEX measurement_y2006m03_logdate ON measurement_y2006m03 (logdate);
...
CREATE INDEX measurement_y2007m11_logdate ON measurement_y2007m11 (logdate);
CREATE INDEX measurement_y2007m12_logdate ON measurement_y2007m12 (logdate);
CREATE INDEX measurement_y2008m01_logdate ON measurement_y2008m01 (logdate);</PRE
><P>
We choose not to add further indexes at this time.
</P
></LI
><LI
><P
> We want our application to be able to say <TT
CLASS="LITERAL"
>INSERT INTO
measurement ...</TT
> and have the data be redirected into the
appropriate partition table. We can arrange that by attaching
a suitable trigger function to the master table.
If data will be added only to the latest partition, we can
use a very simple trigger function:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE OR REPLACE FUNCTION measurement_insert_trigger()
RETURNS TRIGGER AS $$
BEGIN
INSERT INTO measurement_y2008m01 VALUES (NEW.*);
RETURN NULL;
END;
$$
LANGUAGE plpgsql;</PRE
><P>
After creating the function, we create a trigger which
calls the trigger function:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TRIGGER insert_measurement_trigger
BEFORE INSERT ON measurement
FOR EACH ROW EXECUTE PROCEDURE measurement_insert_trigger();</PRE
><P>
We must redefine the trigger function each month so that it always
points to the current partition. The trigger definition does
not need to be updated, however.
</P
><P
> We might want to insert data and have the server automatically
locate the partition into which the row should be added. We
could do this with a more complex trigger function, for example:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE OR REPLACE FUNCTION measurement_insert_trigger()
RETURNS TRIGGER AS $$
BEGIN
IF ( NEW.logdate >= DATE '2006-02-01' AND
NEW.logdate < DATE '2006-03-01' ) THEN
INSERT INTO measurement_y2006m02 VALUES (NEW.*);
ELSIF ( NEW.logdate >= DATE '2006-03-01' AND
NEW.logdate < DATE '2006-04-01' ) THEN
INSERT INTO measurement_y2006m03 VALUES (NEW.*);
...
ELSIF ( NEW.logdate >= DATE '2008-01-01' AND
NEW.logdate < DATE '2008-02-01' ) THEN
INSERT INTO measurement_y2008m01 VALUES (NEW.*);
ELSE
RAISE EXCEPTION 'Date out of range. Fix the measurement_insert_trigger() function!';
END IF;
RETURN NULL;
END;
$$
LANGUAGE plpgsql;</PRE
><P>
The trigger definition is the same as before.
Note that each <TT
CLASS="LITERAL"
>IF</TT
> test must exactly match the
<TT
CLASS="LITERAL"
>CHECK</TT
> constraint for its partition.
</P
><P
> While this function is more complex than the single-month case,
it doesn't need to be updated as often, since branches can be
added in advance of being needed.
</P
><DIV
CLASS="NOTE"
><BLOCKQUOTE
CLASS="NOTE"
><P
><B
>Note: </B
> In practice it might be best to check the newest partition first,
if most inserts go into that partition. For simplicity we have
shown the trigger's tests in the same order as in other parts
of this example.
</P
></BLOCKQUOTE
></DIV
></LI
></OL
><P>
</P
><P
> As we can see, a complex partitioning scheme could require a
substantial amount of DDL. In the above example we would be
creating a new partition each month, so it might be wise to write a
script that generates the required DDL automatically.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="DDL-PARTITIONING-MANAGING-PARTITIONS"
>5.9.3. Managing Partitions</A
></H2
><P
> Normally the set of partitions established when initially
defining the table are not intended to remain static. It is
common to want to remove old partitions of data and periodically
add new partitions for new data. One of the most important
advantages of partitioning is precisely that it allows this
otherwise painful task to be executed nearly instantaneously by
manipulating the partition structure, rather than physically moving large
amounts of data around.
</P
><P
> The simplest option for removing old data is simply to drop the partition
that is no longer necessary:
</P><PRE
CLASS="PROGRAMLISTING"
>DROP TABLE measurement_y2006m02;</PRE
><P>
This can very quickly delete millions of records because it doesn't have
to individually delete every record.
</P
><P
> Another option that is often preferable is to remove the partition from
the partitioned table but retain access to it as a table in its own
right:
</P><PRE
CLASS="PROGRAMLISTING"
>ALTER TABLE measurement_y2006m02 NO INHERIT measurement;</PRE
><P>
This allows further operations to be performed on the data before
it is dropped. For example, this is often a useful time to back up
the data using <TT
CLASS="COMMAND"
>COPY</TT
>, <SPAN
CLASS="APPLICATION"
>pg_dump</SPAN
>, or
similar tools. It might also be a useful time to aggregate data
into smaller formats, perform other data manipulations, or run
reports.
</P
><P
> Similarly we can add a new partition to handle new data. We can create an
empty partition in the partitioned table just as the original partitions
were created above:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TABLE measurement_y2008m02 (
CHECK ( logdate >= DATE '2008-02-01' AND logdate < DATE '2008-03-01' )
) INHERITS (measurement);</PRE
><P>
As an alternative, it is sometimes more convenient to create the
new table outside the partition structure, and make it a proper
partition later. This allows the data to be loaded, checked, and
transformed prior to it appearing in the partitioned table:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE TABLE measurement_y2008m02
(LIKE measurement INCLUDING DEFAULTS INCLUDING CONSTRAINTS);
ALTER TABLE measurement_y2008m02 ADD CONSTRAINT y2008m02
CHECK ( logdate >= DATE '2008-02-01' AND logdate < DATE '2008-03-01' );
\copy measurement_y2008m02 from 'measurement_y2008m02'
-- possibly some other data preparation work
ALTER TABLE measurement_y2008m02 INHERIT measurement;</PRE
><P>
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="DDL-PARTITIONING-CONSTRAINT-EXCLUSION"
>5.9.4. Partitioning and Constraint Exclusion</A
></H2
><P
> <I
CLASS="FIRSTTERM"
>Constraint exclusion</I
> is a query optimization technique
that improves performance for partitioned tables defined in the
fashion described above. As an example:
</P><PRE
CLASS="PROGRAMLISTING"
>SET constraint_exclusion = on;
SELECT count(*) FROM measurement WHERE logdate >= DATE '2008-01-01';</PRE
><P>
Without constraint exclusion, the above query would scan each of
the partitions of the <TT
CLASS="STRUCTNAME"
>measurement</TT
> table. With constraint
exclusion enabled, the planner will examine the constraints of each
partition and try to prove that the partition need not
be scanned because it could not contain any rows meeting the query's
<TT
CLASS="LITERAL"
>WHERE</TT
> clause. When the planner can prove this, it
excludes the partition from the query plan.
</P
><P
> You can use the <TT
CLASS="COMMAND"
>EXPLAIN</TT
> command to show the difference
between a plan with <TT
CLASS="VARNAME"
>constraint_exclusion</TT
> on and a plan
with it off. A typical unoptimized plan for this type of table setup is:
</P><PRE
CLASS="PROGRAMLISTING"
>SET constraint_exclusion = off;
EXPLAIN SELECT count(*) FROM measurement WHERE logdate >= DATE '2008-01-01';
QUERY PLAN
-----------------------------------------------------------------------------------------------
Aggregate (cost=158.66..158.68 rows=1 width=0)
-> Append (cost=0.00..151.88 rows=2715 width=0)
-> Seq Scan on measurement (cost=0.00..30.38 rows=543 width=0)
Filter: (logdate >= '2008-01-01'::date)
-> Seq Scan on measurement_y2006m02 measurement (cost=0.00..30.38 rows=543 width=0)
Filter: (logdate >= '2008-01-01'::date)
-> Seq Scan on measurement_y2006m03 measurement (cost=0.00..30.38 rows=543 width=0)
Filter: (logdate >= '2008-01-01'::date)
...
-> Seq Scan on measurement_y2007m12 measurement (cost=0.00..30.38 rows=543 width=0)
Filter: (logdate >= '2008-01-01'::date)
-> Seq Scan on measurement_y2008m01 measurement (cost=0.00..30.38 rows=543 width=0)
Filter: (logdate >= '2008-01-01'::date)</PRE
><P>
Some or all of the partitions might use index scans instead of
full-table sequential scans, but the point here is that there
is no need to scan the older partitions at all to answer this query.
When we enable constraint exclusion, we get a significantly
cheaper plan that will deliver the same answer:
</P><PRE
CLASS="PROGRAMLISTING"
>SET constraint_exclusion = on;
EXPLAIN SELECT count(*) FROM measurement WHERE logdate >= DATE '2008-01-01';
QUERY PLAN
-----------------------------------------------------------------------------------------------
Aggregate (cost=63.47..63.48 rows=1 width=0)
-> Append (cost=0.00..60.75 rows=1086 width=0)
-> Seq Scan on measurement (cost=0.00..30.38 rows=543 width=0)
Filter: (logdate >= '2008-01-01'::date)
-> Seq Scan on measurement_y2008m01 measurement (cost=0.00..30.38 rows=543 width=0)
Filter: (logdate >= '2008-01-01'::date)</PRE
><P>
</P
><P
> Note that constraint exclusion is driven only by <TT
CLASS="LITERAL"
>CHECK</TT
>
constraints, not by the presence of indexes. Therefore it isn't
necessary to define indexes on the key columns. Whether an index
needs to be created for a given partition depends on whether you
expect that queries that scan the partition will generally scan
a large part of the partition or just a small part. An index will
be helpful in the latter case but not the former.
</P
><P
> The default (and recommended) setting of
<A
HREF="runtime-config-query.html#GUC-CONSTRAINT-EXCLUSION"
>constraint_exclusion</A
> is actually neither
<TT
CLASS="LITERAL"
>on</TT
> nor <TT
CLASS="LITERAL"
>off</TT
>, but an intermediate setting
called <TT
CLASS="LITERAL"
>partition</TT
>, which causes the technique to be
applied only to queries that are likely to be working on partitioned
tables. The <TT
CLASS="LITERAL"
>on</TT
> setting causes the planner to examine
<TT
CLASS="LITERAL"
>CHECK</TT
> constraints in all queries, even simple ones that
are unlikely to benefit.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="DDL-PARTITIONING-ALTERNATIVES"
>5.9.5. Alternative Partitioning Methods</A
></H2
><P
> A different approach to redirecting inserts into the appropriate
partition table is to set up rules, instead of a trigger, on the
master table. For example:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE RULE measurement_insert_y2006m02 AS
ON INSERT TO measurement WHERE
( logdate >= DATE '2006-02-01' AND logdate < DATE '2006-03-01' )
DO INSTEAD
INSERT INTO measurement_y2006m02 VALUES (NEW.*);
...
CREATE RULE measurement_insert_y2008m01 AS
ON INSERT TO measurement WHERE
( logdate >= DATE '2008-01-01' AND logdate < DATE '2008-02-01' )
DO INSTEAD
INSERT INTO measurement_y2008m01 VALUES (NEW.*);</PRE
><P>
A rule has significantly more overhead than a trigger, but the overhead
is paid once per query rather than once per row, so this method might be
advantageous for bulk-insert situations. In most cases, however, the
trigger method will offer better performance.
</P
><P
> Be aware that <TT
CLASS="COMMAND"
>COPY</TT
> ignores rules. If you want to
use <TT
CLASS="COMMAND"
>COPY</TT
> to insert data, you'll need to copy into the correct
partition table rather than into the master. <TT
CLASS="COMMAND"
>COPY</TT
> does fire
triggers, so you can use it normally if you use the trigger approach.
</P
><P
> Another disadvantage of the rule approach is that there is no simple
way to force an error if the set of rules doesn't cover the insertion
date; the data will silently go into the master table instead.
</P
><P
> Partitioning can also be arranged using a <TT
CLASS="LITERAL"
>UNION ALL</TT
>
view, instead of table inheritance. For example,
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE VIEW measurement AS
SELECT * FROM measurement_y2006m02
UNION ALL SELECT * FROM measurement_y2006m03
...
UNION ALL SELECT * FROM measurement_y2007m11
UNION ALL SELECT * FROM measurement_y2007m12
UNION ALL SELECT * FROM measurement_y2008m01;</PRE
><P>
However, the need to recreate the view adds an extra step to adding and
dropping individual partitions of the data set. In practice this
method has little to recommend it compared to using inheritance.
</P
></DIV
><DIV
CLASS="SECT2"
><H2
CLASS="SECT2"
><A
NAME="DDL-PARTITIONING-CAVEATS"
>5.9.6. Caveats</A
></H2
><P
> The following caveats apply to partitioned tables:
<P
></P
></P><UL
><LI
><P
> There is no automatic way to verify that all of the
<TT
CLASS="LITERAL"
>CHECK</TT
> constraints are mutually
exclusive. It is safer to create code that generates
partitions and creates and/or modifies associated objects than
to write each by hand.
</P
></LI
><LI
><P
> 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 <TT
CLASS="COMMAND"
>UPDATE</TT
> that attempts
to do that will fail because of the <TT
CLASS="LITERAL"
>CHECK</TT
> 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.
</P
></LI
><LI
><P
> If you are using manual <TT
CLASS="COMMAND"
>VACUUM</TT
> or
<TT
CLASS="COMMAND"
>ANALYZE</TT
> commands, don't forget that
you need to run them on each partition individually. A command like:
</P><PRE
CLASS="PROGRAMLISTING"
>ANALYZE measurement;</PRE
><P>
will only process the master table.
</P
></LI
></UL
><P>
</P
><P
> The following caveats apply to constraint exclusion:
<P
></P
></P><UL
><LI
><P
> Constraint exclusion only works when the query's <TT
CLASS="LITERAL"
>WHERE</TT
>
clause contains constants (or externally supplied parameters).
For example, a comparison against a non-immutable function such as
<CODE
CLASS="FUNCTION"
>CURRENT_TIMESTAMP</CODE
> cannot be optimized, since the
planner cannot know which partition the function value might fall
into at run time.
</P
></LI
><LI
><P
> Keep the partitioning constraints simple, else the planner may not be
able to prove that partitions don't need to be visited. Use simple
equality conditions for list partitioning, or simple
range tests for range partitioning, as illustrated in the preceding
examples. A good rule of thumb is that partitioning constraints should
contain only comparisons of the partitioning column(s) to constants
using B-tree-indexable operators.
</P
></LI
><LI
><P
> 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. Partitioning using
these techniques will work well with up to perhaps a hundred partitions;
don't try to use many thousands of partitions.
</P
></LI
></UL
><P>
</P
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