</indexterm>
<para>
- <productname>PostgreSQL</> can devise query plans which can leverage
+ <productname>PostgreSQL</> can devise query plans that can leverage
multiple CPUs in order to answer queries faster. This feature is known
as parallel query. Many queries cannot benefit from parallel query, either
due to limitations of the current implementation or because there is no
- imaginable query plan which is any faster than the serial query plan.
+ imaginable query plan that is any faster than the serial query plan.
However, for queries that can benefit, the speedup from parallel query
is often very significant. Many queries can run more than twice as fast
when using parallel query, and some queries can run four times faster or
<para>
When the optimizer determines that parallel query is the fastest execution
- strategy for a particular query, it will create a query plan which includes
+ strategy for a particular query, it will create a query plan that includes
a <firstterm>Gather node</firstterm>. Here is a simple example:
<screen>
<para>
<link linkend="using-explain">Using EXPLAIN</>, you can see the number of
workers chosen by the planner. When the <literal>Gather</> node is reached
- during query execution, the process which is implementing the user's
+ during query execution, the process that is implementing the user's
session will request a number of <link linkend="bgworker">background
worker processes</link> equal to the number
of workers chosen by the planner. The total number of background
</para>
<para>
- Every background worker process which is successfully started for a given
- parallel query will execute the portion of the plan which is a descendent
+ Every background worker process that is successfully started for a given
+ parallel query will execute the portion of the plan that is a descendent
of the <literal>Gather</> node. The leader will also execute that portion
of the plan, but it has an additional responsibility: it must also read
all of the tuples generated by the workers. When the parallel portion of
Conversely, when the parallel portion of the plan generates a large number
of tuples, the leader may be almost entirely occupied with reading the
tuples generated by the workers and performing any further processing
- steps which are required by plan nodes above the level of the
+ steps that are required by plan nodes above the level of the
<literal>Gather</literal> node. In such cases, the leader will do very
little of the work of executing the parallel portion of the plan.
</para>
<title>When Can Parallel Query Be Used?</title>
<para>
- There are several settings which can cause the query planner not to
+ There are several settings that can cause the query planner not to
generate a parallel query plan under any circumstances. In order for
any parallel query plans whatsoever to be generated, the following
settings must be configured as indicated.
<listitem>
<para>
<xref linkend="guc-max-parallel-workers-per-gather"> must be set to a
- value which is greater than zero. This is a special case of the more
+ value that is greater than zero. This is a special case of the more
general principle that no more workers should be used than the number
configured via <varname>max_parallel_workers_per_gather</varname>.
</para>
The query writes any data or locks any database rows. If a query
contains a data-modifying operation either at the top level or within
a CTE, no parallel plans for that query will be generated. This is a
- limitation of the current implementation which could be lifted in a
+ limitation of the current implementation that could be lifted in a
future release.
</para>
</listitem>
than normal but would produce incorrect results. Instead, the parallel
portion of the plan must be what is known internally to the query
optimizer as a <firstterm>partial plan</>; that is, it must be constructed
- so that each process which executes the plan will generate only a
+ so that each process that executes the plan will generate only a
subset of the output rows in such a way that each required output row
is guaranteed to be generated by exactly one of the cooperating processes.
</para>
<para>
Because the <literal>Finalize Aggregate</> node runs on the leader
- process, queries which produce a relatively large number of groups in
+ process, queries that produce a relatively large number of groups in
comparison to the number of input rows will appear less favorable to the
query planner. For example, in the worst-case scenario the number of
groups seen by the <literal>Finalize Aggregate</> node could be as many as
- the number of input rows which were seen by all worker processes in the
+ the number of input rows that were seen by all worker processes in the
<literal>Partial Aggregate</> stage. For such cases, there is clearly
going to be no performance benefit to using parallel aggregation. The
query planner takes this into account during the planning process and is
If a query that is expected to do so does not produce a parallel plan,
you can try reducing <xref linkend="guc-parallel-setup-cost"> or
<xref linkend="guc-parallel-tuple-cost">. Of course, this plan may turn
- out to be slower than the serial plan which the planner preferred, but
+ out to be slower than the serial plan that the planner preferred, but
this will not always be the case. If you don't get a parallel
plan even with very small values of these settings (e.g., after setting
them both to zero), there may be some reason why the query planner is
<para>
The planner classifies operations involved in a query as either
<firstterm>parallel safe</>, <firstterm>parallel restricted</>,
- or <firstterm>parallel unsafe</>. A parallel safe operation is one which
+ or <firstterm>parallel unsafe</>. A parallel safe operation is one that
does not conflict with the use of parallel query. A parallel restricted
- operation is one which cannot be performed in a parallel worker, but which
+ operation is one that cannot be performed in a parallel worker, but that
can be performed in the leader while parallel query is in use. Therefore,
parallel restricted operations can never occur below a <literal>Gather</>
node, but can occur elsewhere in a plan which contains a
- <literal>Gather</> node. A parallel unsafe operation is one which cannot
+ <literal>Gather</> node. A parallel unsafe operation is one that cannot
be performed while parallel query is in use, not even in the leader.
- When a query contains anything which is parallel unsafe, parallel query
+ When a query contains anything that is parallel unsafe, parallel query
is completely disabled for that query.
</para>
<listitem>
<para>
Scans of foreign tables, unless the foreign data wrapper has
- an <literal>IsForeignScanParallelSafe</> API which indicates otherwise.
+ an <literal>IsForeignScanParallelSafe</> API that indicates otherwise.
</para>
</listitem>
<para>
The planner cannot automatically determine whether a user-defined
function or aggregate is parallel safe, parallel restricted, or parallel
- unsafe, because this would require predicting every operation which the
+ unsafe, because this would require predicting every operation that the
function could possibly perform. In general, this is equivalent to the
Halting Problem and therefore impossible. Even for simple functions
where it could conceivably be done, we do not try, since this would be expensive
<para>
Functions and aggregates must be marked <literal>PARALLEL UNSAFE</> if
they write to the database, access sequences, change the transaction state
- even temporarily (e.g., a PL/pgsql function which establishes an
+ even temporarily (e.g., a PL/pgsql function that establishes an
<literal>EXCEPTION</> block to catch errors), or make persistent changes to
settings. Similarly, functions must be marked <literal>PARALLEL
RESTRICTED</> if they access temporary tables, client connection state,
- cursors, prepared statements, or miscellaneous backend-local state which
+ cursors, prepared statements, or miscellaneous backend-local state that
the system cannot synchronize across workers. For example,
<literal>setseed</> and <literal>random</> are parallel restricted for
this last reason.
</para>
<para>
- If a function executed within a parallel worker acquires locks which are
+ If a function executed within a parallel worker acquires locks that are
not held by the leader, for example by querying a table not referenced in
the query, those locks will be released at worker exit, not end of
- transaction. If you write a function which does this, and this behavior
+ transaction. If you write a function that does this, and this behavior
difference is important to you, mark such functions as
<literal>PARALLEL RESTRICTED</literal>
to ensure that they execute only in the leader.
projects / thirdparty / postgresql.git / commitdiff
