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

Author: michaelpq

doc/src/sgml/parallel.sgml

@@ -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>
@@ -144,8 +144,8 @@ EXPLAIN SELECT * FROM pgbench_accounts WHERE filler LIKE '%x%';
         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.  As an
-        exception, the following commands which create a new table and populate
-        it can use a parallel plan for the underlying <literal>SELECT</literal>
+        exception, the following commands, which create a new table and populate
+        it, can use a parallel plan for the underlying <literal>SELECT</literal>
         part of the query:
 
         <itemizedlist>
@@ -255,7 +255,7 @@ 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
@@ -365,11 +365,11 @@ 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
@@ -425,7 +425,7 @@ 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
@@ -446,7 +446,7 @@ 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
@@ -473,15 +473,15 @@ 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>
 
@@ -505,7 +505,7 @@ 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>
 
@@ -517,7 +517,7 @@ 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>
@@ -528,7 +528,7 @@ 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
@@ -546,11 +546,11 @@ 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.
@@ -568,10 +568,10 @@ 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.