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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 <em class="firstterm">Gather</em> or <em class="firstterm">Gather Merge</em>
node. Here is a simple example:
</p><pre class="screen">
EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
QUERY PLAN
-------------------------------------------------------------------------------------
Gather (cost=1000.00..217018.43 rows=1 width=97)
Workers Planned: 2
-> Parallel Seq Scan on pgbench_accounts (cost=0.00..216018.33 rows=1 width=97)
Filter: (filler ~~ '%x%'::text)
(4 rows)
</pre><p>
</p><p>
In all cases, the <code class="literal">Gather</code> or
<code class="literal">Gather Merge</code> node will have exactly one
child plan, which is the portion of the plan that will be executed in
parallel. If the <code class="literal">Gather</code> or <code class="literal">Gather Merge</code> 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 <code class="literal">Gather</code> node itself; since that plan node is a child of the
<code class="literal">Gather</code> node, it will run in parallel.
</p><p>
<a class="link" href="using-explain.html" title="14.1. Using EXPLAIN">Using EXPLAIN</a>, you can see the number of
workers chosen by the planner. When the <code class="literal">Gather</code> node is reached
during query execution, the process which is implementing the user's
session will request a number of <a class="link" href="bgworker.html" title="Chapter 48. Background Worker Processes">background
worker processes</a> equal to the number
of workers chosen by the planner. The number of background workers that
the planner will consider using is limited to at most
<a class="xref" href="runtime-config-resource.html#GUC-MAX-PARALLEL-WORKERS-PER-GATHER">max_parallel_workers_per_gather</a>. The total number
of background workers that can exist at any one time is limited by both
<a class="xref" href="runtime-config-resource.html#GUC-MAX-WORKER-PROCESSES">max_worker_processes</a> and
<a class="xref" href="runtime-config-resource.html#GUC-MAX-PARALLEL-WORKERS">max_parallel_workers</a>. 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, consider increasing
<code class="varname">max_worker_processes</code> and <code class="varname">max_parallel_workers</code>
so that more workers can be run simultaneously or alternatively reducing
<code class="varname">max_parallel_workers_per_gather</code> so that the planner
requests fewer workers.
</p><p>
Every background worker process which is successfully started for a given
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 <code class="literal">Gather</code> node or
<code class="literal">Gather Merge</code> node. In such cases, the leader will
do very little of the work of executing the parallel portion of the plan.
</p><p>
When the node at the top of the parallel portion of the plan is
<code class="literal">Gather Merge</code> rather than <code class="literal">Gather</code>, 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, <code class="literal">Gather</code> reads tuples
from the workers in whatever order is convenient, destroying any sort
order that may have existed.
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