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Make some more improvements to parallel query documentation.

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Many places that mentioned only Gather should also mention Gather
Merge, or should be phrased in a more neutral way.  Be more clear
about the fact that max_parallel_workers_per_gather affects the number
of workers the planner may want to use.  Fix a typo.  Explain how
Gather Merge works.  Adjust wording around parallel scans to be a bit
more clear.  Adjust wording around parallel-restricted operations for
the fact that uncorrelated subplans are no longer restricted.

Patch by me, reviewed by Erik Rijkers

Discussion: http://postgr.es/m/CA+TgmoZsTjgVGn=ei5ht-1qGFKy_m1VgB3d8+Rg304hz91N5ww@mail.gmail.com
This commit is contained in:
Robert Haas 2017-08-10 13:22:31 -04:00
parent e694010758
commit c1ef4e5cdb
2 changed files with 58 additions and 43 deletions
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4
doc/src/sgml/config.sgml
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@ -2050,8 +2050,8 @@ include_dir 'conf.d'
<listitem>
<para>
Sets the maximum number of workers that can be started by a single
<literal>Gather</literal> node. Parallel workers are taken from the
pool of processes established by
<literal>Gather</literal> or <literal>Gather Merge</literal> node.
Parallel workers are taken from the pool of processes established by
<xref linkend="guc-max-worker-processes">, limited by
<xref linkend="guc-max-parallel-workers">. Note that the requested
number of workers may not actually be available at run time. If this

97
doc/src/sgml/parallel.sgml
View File

@ -28,7 +28,8 @@
<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
a <firstterm>Gather node</firstterm>. Here is a simple example:
a <firstterm>Gather</firstterm> or <firstterm>Gather Merge</firstterm>
node. Here is a simple example:
<screen>
EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
@ -43,15 +44,16 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
</para>
<para>
In all cases, the <literal>Gather</literal> node will have exactly one
In all cases, the <literal>Gather</literal> or
<literal>Gather Merge</literal> node will have exactly one
child plan, which is the portion of the plan that will be executed in
parallel. If the <literal>Gather</> node is at the very top of the plan
tree, then the entire query will execute in parallel. If it is somewhere
else in the plan tree, then only the portion of the plan below it will run
in parallel. In the example above, the query accesses only one table, so
there is only one plan node other than the <literal>Gather</> node itself;
since that plan node is a child of the <literal>Gather</> node, it will
run in parallel.
parallel. If the <literal>Gather</> or <literal>Gather Merge</> node is
at the very top of the plan tree, then the entire query will execute in
parallel. If it is somewhere else in the plan tree, then only the portion
of the plan below it will run in parallel. In the example above, the
query accesses only one table, so there is only one plan node other than
the <literal>Gather</> node itself; since that plan node is a child of the
<literal>Gather</> node, it will run in parallel.
</para>
<para>
@ -60,35 +62,47 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
during query execution, the process which 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
workers that can exist at any one time is limited by both
of workers chosen by the planner. The number of background workers that
the planner will consider using is limited to at most
<xref linkend="guc-max-parallel-workers-per-gather">. The total number
of background workers that can exist at any one time is limited by both
<xref linkend="guc-max-worker-processes"> and
<xref linkend="guc-max-parallel-workers">, so it is possible for a
<xref linkend="guc-max-parallel-workers">. Therefore, it is possible for a
parallel query to run with fewer workers than planned, or even with
no workers at all. The optimal plan may depend on the number of workers
that are available, so this can result in poor query performance. If this
occurrence is frequent, considering increasing
occurrence is frequent, consider increasing
<varname>max_worker_processes</> and <varname>max_parallel_workers</>
so that more workers can be run simultaneously or alternatively reducing
<xref linkend="guc-max-parallel-workers-per-gather"> so that the planner
<varname>max_parallel_workers_per_gather</varname> so that the planner
requests fewer workers.
</para>
<para>
Every background worker process which is successfully started for a given
parallel query will execute the portion of the plan below
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
the plan generates only a small number of tuples, the leader will often
behave very much like an additional worker, speeding up query execution.
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
<literal>Gather</literal> node. In such cases, the leader will do very
little of the work of executing the parallel portion of the plan.
parallel query will execute the parallel portion of the plan. 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 the plan generates only a small
number of tuples, the leader will often behave very much like an additional
worker, speeding up query execution. 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 <literal>Gather</literal> node or
<literal>Gather Merge</literal> node. In such cases, the leader will
do very little of the work of executing the parallel portion of the plan.
</para>
<para>
When the node at the top of the parallel portion of the plan is
<literal>Gather Merge</> rather than <literal>Gather</>, it indicates that
each process executing the parallel portion of the plan is producing
tuples in sorted order, and that the leader is performing an
order-preserving merge. In contrast, <literal>Gather</> reads tuples
from the workers in whatever order is convenient, destroying any sort
order that may have existed.
</para>
</sect1>
<sect1 id="when-can-parallel-query-be-used">
@ -221,9 +235,9 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
send such a message, this can only occur when using a client that
does not rely on libpq. If this is a frequent
occurrence, it may be a good idea to set
<xref linkend="guc-max-parallel-workers-per-gather"> in sessions
where it is likely, so as to avoid generating query plans that may
be suboptimal when run serially.
<xref linkend="guc-max-parallel-workers-per-gather"> to zero in
sessions where it is likely, so as to avoid generating query plans
that may be suboptimal when run serially.
</para>
</listitem>
@ -262,6 +276,8 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
so that each process which 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.
Generally, this means that the scan on the driving table of the query
must be a parallel-aware scan.
</para>
<sect2 id="parallel-scans">
@ -302,9 +318,8 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
</listitem>
</itemizedlist>
Only the scan types listed above may be used for a scan on the driving
table within a parallel plan. Other scan types, such as parallel scans of
non-btree indexes, may be supported in the future.
Other scan types, such as scans of non-btree indexes, may support
parallel scans in the future.
</para>
</sect2>
@ -343,10 +358,10 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
the query performs an aggregation step, producing a partial result for
each group of which that process is aware. This is reflected in the plan
as a <literal>Partial Aggregate</> node. Second, the partial results are
transferred to the leader via the <literal>Gather</> node. Finally, the
leader re-aggregates the results across all workers in order to produce
the final result. This is reflected in the plan as a
<literal>Finalize Aggregate</> node.
transferred to the leader via <literal>Gather</> or <literal>Gather
Merge</>. Finally, the leader re-aggregates the results across all
workers in order to produce the final result. This is reflected in the
plan as a <literal>Finalize Aggregate</> node.
</para>
<para>
@ -416,8 +431,8 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
operation is one which cannot be performed in a parallel worker, but which
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
or <literal>Gather Merge</> node, but can occur elsewhere in a plan which
contains such a node. A parallel unsafe operation is one which 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
is completely disabled for that query.
@ -449,7 +464,7 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
<listitem>
<para>
Access to an <literal>InitPlan</> or <literal>SubPlan</>.
Access to an <literal>InitPlan</> or correlated <literal>SubPlan</>.
</para>
</listitem>
</itemizedlist>
@ -514,8 +529,8 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
parallel-restricted functions or aggregates involved in the query in
order to obtain a superior plan. So, for example, if a <literal>WHERE</>
clause applied to a particular table is parallel restricted, the query
planner will not consider placing the scan of that table below a
<literal>Gather</> node. In some cases, it would be
planner will not consider performing a scan of that table in the parallel
portion of a plan. In some cases, it would be
possible (and perhaps even efficient) to include the scan of that table in
the parallel portion of the query and defer the evaluation of the
<literal>WHERE</> clause so that it happens above the <literal>Gather</>