Improve contrib/pg_stat_statements to lump "similar" queries together.

pg_stat_statements now hashes selected fields of the analyzed parse tree
to assign a "fingerprint" to each query, and groups all queries with the
same fingerprint into a single entry in the pg_stat_statements view.
In practice it is expected that queries with the same fingerprint will be
equivalent except for values of literal constants.  To make the display
more useful, such constants are replaced by "?" in the displayed query
strings.

This mechanism currently supports only optimizable queries (SELECT,
INSERT, UPDATE, DELETE).  Utility commands are still matched on the
basis of their literal query strings.

There remain some open questions about how to deal with utility statements
that contain optimizable queries (such as EXPLAIN and SELECT INTO) and how
to deal with expiring speculative hashtable entries that are made to save
the normalized form of a query string.  However, fixing these issues should
require only localized changes, and since there are other open patches
involving contrib/pg_stat_statements, it seems best to go ahead and commit
what we've got.

Peter Geoghegan, reviewed by Daniel Farina

doc/src/sgml/pgstatstatements.sgml

@@ -25,7 +25,7 @@
   <para>
    The statistics gathered by the module are made available via a system view
    named <structname>pg_stat_statements</>.  This view contains one row for
-   each distinct query text, database ID, and user ID (up to the maximum
+   each distinct query, database ID, and user ID (up to the maximum
    number of distinct statements that the module can track).  The columns
    of the view are shown in <xref linkend="pgstatstatements-columns">.
   </para>
@@ -61,7 +61,7 @@
       <entry><structfield>query</structfield></entry>
       <entry><type>text</type></entry>
       <entry></entry>
-      <entry>Text of the statement (up to <xref linkend="guc-track-activity-query-size"> bytes)</entry>
+      <entry>Text of a representative statement (up to <xref linkend="guc-track-activity-query-size"> bytes)</entry>
      </row>
 
      <row>
@@ -195,10 +195,38 @@
   </para>
 
   <para>
-   Note that statements are considered the same if they have the same text,
-   regardless of the values of any out-of-line parameters used in the
-   statement.  Using out-of-line parameters will help to group statements
-   together and may make the statistics more useful.
+   Plannable queries (that is, <command>SELECT</>, <command>INSERT</>,
+   <command>UPDATE</>, and <command>DELETE</>) are combined into a single
+   <structname>pg_stat_statements</> entry whenever they have identical query
+   structures according to an internal hash calculation.  Typically, two
+   queries will be considered the same for this purpose if they are
+   semantically equivalent except for the values of literal constants
+   appearing in the query.  Utility commands (that is, all other commands)
+   are compared strictly on the basis of their textual query strings, however.
+  </para>
+
+  <para>
+   When a constant's value has been ignored for purposes of matching the
+   query to other queries, the constant is replaced by <literal>?</literal>
+   in the <structname>pg_stat_statements</> display.  The rest of the query
+   text is that of the first query that had the particular hash value
+   associated with the <structname>pg_stat_statements</> entry.
+  </para>
+
+  <para>
+   In some cases, queries with visibly different texts might get merged into a
+   single <structname>pg_stat_statements</> entry.  Normally this will happen
+   only for semantically equivalent queries, but there is a small chance of
+   hash collisions causing unrelated queries to be merged into one entry.
+   (This cannot happen for queries belonging to different users or databases,
+   however.)
+  </para>
+
+  <para>
+   Since the hash value is computed on the post-parse-analysis representation
+   of the queries, the opposite is also possible: queries with identical texts
+   might appear as separate entries, if they have different meanings as a
+   result of factors such as different <varname>search_path</> settings.
   </para>
  </sect2>
 
@@ -329,20 +357,20 @@ pg_stat_statements.track = all
 bench=# SELECT pg_stat_statements_reset();
 
 $ pgbench -i bench
-$ pgbench -c10 -t300 -M prepared bench
+$ pgbench -c10 -t300 bench
 
 bench=# \x
 bench=# SELECT query, calls, total_time, rows, 100.0 * shared_blks_hit /
                nullif(shared_blks_hit + shared_blks_read, 0) AS hit_percent
           FROM pg_stat_statements ORDER BY total_time DESC LIMIT 5;
 -[ RECORD 1 ]---------------------------------------------------------------------
-query       | UPDATE pgbench_branches SET bbalance = bbalance + $1 WHERE bid = $2;
+query       | UPDATE pgbench_branches SET bbalance = bbalance + ? WHERE bid = ?;
 calls       | 3000
 total_time  | 9.60900100000002
 rows        | 2836
 hit_percent | 99.9778970000200936
 -[ RECORD 2 ]---------------------------------------------------------------------
-query       | UPDATE pgbench_tellers SET tbalance = tbalance + $1 WHERE tid = $2;
+query       | UPDATE pgbench_tellers SET tbalance = tbalance + ? WHERE tid = ?;
 calls       | 3000
 total_time  | 8.015156
 rows        | 2990
@@ -354,7 +382,7 @@ total_time  | 0.310624
 rows        | 100000
 hit_percent | 0.30395136778115501520
 -[ RECORD 4 ]---------------------------------------------------------------------
-query       | UPDATE pgbench_accounts SET abalance = abalance + $1 WHERE aid = $2;
+query       | UPDATE pgbench_accounts SET abalance = abalance + ? WHERE aid = ?;
 calls       | 3000
 total_time  | 0.271741999999997
 rows        | 3000