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doc: Replace some uses of "which" by "that" in parallel.sgml 

This makes the documentation more accurate grammatically.

Author: Elena Indrupskaya
Discussion: https://postgr.es/m/1c994b3d-951e-59bb-1ac2-7b9221c0e4cf@postgrespro.ru
Backpatch-through: 9.6
This commit is contained in:
Michael Paquier 2021-09-02 11:36:01 +09:00
parent b51985d8a0
commit e976cc4a79
1 changed files with 26 additions and 26 deletions
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52
doc/src/sgml/parallel.sgml
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@ -8,11 +8,11 @@
</indexterm>
<para>
<productname>PostgreSQL</productname> can devise query plans which can leverage
<productname>PostgreSQL</productname> 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
@ -27,7 +27,7 @@
<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</firstterm> or <firstterm>Gather Merge</firstterm>
node. Here is a simple example:
@ -59,7 +59,7 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
<para>
<link linkend="using-explain">Using EXPLAIN</link>, you can see the number of
workers chosen by the planner. When the <literal>Gather</literal> 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 number of background workers that
@ -79,7 +79,7 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
</para>
<para>
Every background worker process which is successfully started for a given
Every background worker process that 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
@ -88,7 +88,7 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
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
performing any further processing steps that 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.
@ -109,7 +109,7 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
<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.
@ -119,7 +119,7 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
<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>
@ -145,7 +145,7 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
contains a data-modifying operation either at the top level or within
a CTE, no parallel plans for that query will be generated. As an
exception, the commands <literal>CREATE TABLE ... AS</literal>, <literal>SELECT
INTO</literal>, and <literal>CREATE MATERIALIZED VIEW</literal> which create a new
INTO</literal>, and <literal>CREATE MATERIALIZED VIEW</literal> that create a new
table and populate it can use a parallel plan.
</para>
</listitem>
@ -240,7 +240,7 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
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</firstterm>; 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.
Generally, this means that the scan on the driving table of the query
@ -350,11 +350,11 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
<para>
Because the <literal>Finalize Aggregate</literal> 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</literal> 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</literal> 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
@ -410,7 +410,7 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
involve appending multiple results sets can therefore achieve
coarse-grained parallelism even when efficient partial plans are not
available. For example, consider a query against a partitioned table
which can only be implemented efficiently by using an index that does
that can only be implemented efficiently by using an index that does
not support parallel scans. The planner might choose a <literal>Parallel
Append</literal> of regular <literal>Index Scan</literal> plans; each
individual index scan would have to be executed to completion by a single
@ -431,7 +431,7 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
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
@ -458,15 +458,15 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
<para>
The planner classifies operations involved in a query as either
<firstterm>parallel safe</firstterm>, <firstterm>parallel restricted</firstterm>,
or <firstterm>parallel unsafe</firstterm>. A parallel safe operation is one which
or <firstterm>parallel unsafe</firstterm>. 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</literal>
or <literal>Gather Merge</literal> node, but can occur elsewhere in a plan which
contains such a node. A parallel unsafe operation is one which cannot
or <literal>Gather Merge</literal> node, but can occur elsewhere in a plan that
contains such a 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>
@ -490,7 +490,7 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
<listitem>
<para>
Scans of foreign tables, unless the foreign data wrapper has
an <literal>IsForeignScanParallelSafe</literal> API which indicates otherwise.
an <literal>IsForeignScanParallelSafe</literal> API that indicates otherwise.
</para>
</listitem>
@ -502,7 +502,7 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
<listitem>
<para>
Plan nodes which reference a correlated <literal>SubPlan</literal>.
Plan nodes that reference a correlated <literal>SubPlan</literal>.
</para>
</listitem>
</itemizedlist>
@ -513,7 +513,7 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
<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
@ -531,11 +531,11 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
<para>
Functions and aggregates must be marked <literal>PARALLEL UNSAFE</literal> 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</literal> block to catch errors), or make persistent changes to
settings. Similarly, functions must be marked <literal>PARALLEL
RESTRICTED</literal> 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</literal> and <literal>random</literal> are parallel restricted for
this last reason.
@ -553,10 +553,10 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
</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.