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Enable parallel query by default.

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Change max_parallel_degree default from 0 to 2.  It is possible that
this is not a good idea, or that we should go with 1 worker rather
than 2, but we won't find out without trying it.  Along the way,
reword the documentation for max_parallel_degree a little bit to
hopefully make it more clear.

Discussion: 20160420174631.3qjjhpwsvvx5bau5@alap3.anarazel.de
This commit is contained in:
Robert Haas
2016-04-26 08:31:38 -04:00
parent b7351ced42
commit 77cd477c4b
4 changed files with 11 additions and 7 deletions
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12
doc/src/sgml/config.sgml
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@@ -2005,10 +2005,14 @@ include_dir 'conf.d'
</term> </term>
<listitem> <listitem>
<para> <para>
Sets the maximum degree of parallelism for an individual parallel Sets the maximum number of workers that can be started for an
operation. Note that the requested number of workers may not actually individual parallel operation. Parallel workers are taken from the
be available at runtime. Parallel workers are taken from the pool pool of processes established by
of processes established by <xref linkend="guc-max-worker-processes">. <xref linkend="guc-max-worker-processes">. Note that the requested
number of workers may not actually be available at runtime. If this
occurs, the plan will run with fewer workers than expected, which may
be inefficient. The default value is 2. Setting this value to 0
disables parallel query.
</para> </para>
</listitem> </listitem>
</varlistentry> </varlistentry>

2
src/backend/optimizer/path/costsize.c
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@@ -113,7 +113,7 @@ int effective_cache_size = DEFAULT_EFFECTIVE_CACHE_SIZE;
Cost disable_cost = 1.0e10; Cost disable_cost = 1.0e10;
int max_parallel_degree = 0; int max_parallel_degree = 2;
bool enable_seqscan = true; bool enable_seqscan = true;
bool enable_indexscan = true; bool enable_indexscan = true;

2
src/backend/utils/misc/guc.c
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@@ -2662,7 +2662,7 @@ static struct config_int ConfigureNamesInt[] =
NULL NULL
}, },
&max_parallel_degree, &max_parallel_degree,
0, 0, MAX_BACKENDS, 2, 0, MAX_BACKENDS,
NULL, NULL, NULL NULL, NULL, NULL
}, },

2
src/backend/utils/misc/postgresql.conf.sample
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@@ -167,7 +167,7 @@
#effective_io_concurrency = 1 # 1-1000; 0 disables prefetching #effective_io_concurrency = 1 # 1-1000; 0 disables prefetching
#max_worker_processes = 8 #max_worker_processes = 8
#max_parallel_degree = 0 # max number of worker processes per node #max_parallel_degree = 2 # max number of worker processes per node
#old_snapshot_threshold = -1 # 1min-60d; -1 disables; 0 is immediate #old_snapshot_threshold = -1 # 1min-60d; -1 disables; 0 is immediate
# (change requires restart) # (change requires restart)
#backend_flush_after = 0 # 0 disables, #backend_flush_after = 0 # 0 disables,