Improve docs for EXPLAIN (IO)

Commit 681daed931 introduced a new EXPLAIN option "IO", but the docs
did not explain what information was added to the output. Expand the
description a little bit, similarly to the other EXPLAIN options.

While at it, fix a typo in the first sentence.

Author: Tomas Vondra <tomas@vondra.me>

diff --git a/doc/src/sgml/ref/explain.sgml b/doc/src/sgml/ref/explain.sgml
index e95e19081e189eea59caa04bcc9137885a0fffa1..f38d31e106a3999a7e6749a3d9ee919bef2311c1 100644 (file)
--- a/doc/src/sgml/ref/explain.sgml
+++ b/doc/src/sgml/ref/explain.sgml
@@ -303,7 +303,14 @@ ROLLBACK;
     <term><literal>IO</literal></term>
     <listitem>
      <para>
-      Include information on I/O performed by scan nodes proving such information.
+      Include information on I/O performed by scan nodes providing such
+      information.
+      Specifically, include information about the prefetch queue (the average
+      and the maximum distance, and the queue capacity), and information about
+      issued I/O requests (the total number of requests, the average request
+      size, the number of I/O waits, and the average number of concurrent I/O
+      requests). The request size is reported in blocks.
+      In text format, only non-zero values are printed.
       This parameter may only be used when <literal>ANALYZE</literal> is also
       enabled.  It defaults to <literal>FALSE</literal>.
      </para>