Replace min_parallel_relation_size with two new GUCs.

When min_parallel_relation_size was added, the only supported type
of parallel scan was a parallel sequential scan, but there are
pending patches for parallel index scan, parallel index-only scan,
and parallel bitmap heap scan.  Those patches introduce two new
types of complications: first, what's relevant is not really the
total size of the relation but the portion of it that we will scan;
and second, index pages and heap pages shouldn't necessarily be
treated in exactly the same way.  Typically, the number of index
pages will be quite small, but that doesn't necessarily mean that
a parallel index scan can't pay off.

Therefore, we introduce min_parallel_table_scan_size, which works
out a degree of parallelism for scans based on the number of table
pages that will be scanned (and which is therefore equivalent to
min_parallel_relation_size for parallel sequential scans) and also
min_parallel_index_scan_size which can be used to work out a degree
of parallelism based on the number of index pages that will be
scanned.

Amit Kapila and Robert Haas

Discussion: http://postgr.es/m/CAA4eK1KowGSYYVpd2qPpaPPA5R90r++QwDFbrRECTE9H_HvpOg@mail.gmail.com
Discussion: http://postgr.es/m/CAA4eK1+TnM4pXQbvn7OXqam+k_HZqb0ROZUMxOiL6DWJYCyYow@mail.gmail.com

2 files changed, 28 insertions, 7 deletions

diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index dc63d7d5e40..95afc2c483d 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -3835,16 +3835,37 @@ ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="
       </listitem>
      </varlistentry>
 
-     <varlistentry id="guc-min-parallel-relation-size" xreflabel="min_parallel_relation_size">
-      <term><varname>min_parallel_relation_size</varname> (<type>integer</type>)
+     <varlistentry id="guc-min-parallel-table-scan-size" xreflabel="min_parallel_table_scan_size">
+      <term><varname>min_parallel_table_scan_size</varname> (<type>integer</type>)
       <indexterm>
-       <primary><varname>min_parallel_relation_size</> configuration parameter</primary>
+       <primary><varname>min_parallel_table_scan_size</> configuration parameter</primary>
       </indexterm>
       </term>
       <listitem>
        <para>
-        Sets the minimum size of relations to be considered for parallel scan.
-        The default is 8 megabytes (<literal>8MB</>).
+        Sets the minimum amount of table data that must be scanned in order
+        for a parallel scan to be considered.  For a parallel sequential scan,
+        the amount of table data scanned is always equal to the size of the
+        table, but when indexes are used the amount of table data
+        scanned will normally be less.  The default is 8
+        megabytes (<literal>8MB</>).
+       </para>
+      </listitem>
+     </varlistentry>
+
+     <varlistentry id="guc-min-parallel-index-scan-size" xreflabel="min_parallel_index_scan_size">
+      <term><varname>min_parallel_index_scan_size</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>min_parallel_index_scan_size</> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        Sets the minimum amount of index data that must be scanned in order
+        for a parallel scan to be considered.  Note that a parallel index scan
+        typically won't touch the entire index; it is the number of pages
+        which the planner believes will actually be touched by the scan which
+        is relevant.  The default is 512 kilobytes (<literal>512kB</>).
        </para>
       </listitem>
      </varlistentry>
diff --git a/doc/src/sgml/release-9.6.sgml b/doc/src/sgml/release-9.6.sgml
index bffcaac46e7..02cc8c9003c 100644
--- a/doc/src/sgml/release-9.6.sgml
+++ b/doc/src/sgml/release-9.6.sgml
@@ -2407,8 +2407,8 @@ and many others in the same vein
         is available through other new configuration parameters
         <xref linkend="guc-force-parallel-mode">,
         <xref linkend="guc-parallel-setup-cost">, <xref
-        linkend="guc-parallel-tuple-cost">, and <xref
-        linkend="guc-min-parallel-relation-size">.
+        linkend="guc-parallel-tuple-cost">, and
+        <literal>min_parallel_relation_size</literal>.
        </para>
       </listitem>