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Show ignored constants as "$N" rather than "?" in pg_stat_statements.

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The trouble with the original choice here is that "?" is a valid (and
indeed used) operator name, so that you could end up with ambiguous
statement texts like "SELECT ? ? ?".  With this patch, you instead
see "SELECT $1 ? $2", which seems significantly more readable.
The numbers used for this purpose begin after the last actual $N parameter
in the particular query.  The conflict with external parameters has its own
potential for confusion of course, but it was agreed to be an improvement
over the previous behavior.

Lukas Fittl

Discussion: https://postgr.es/m/CAP53PkxeaCuwYmF-A4J5z2-qk5fYFo5_NH3gpXGJJBxv1DMwEw@mail.gmail.com
This commit is contained in:
Tom Lane
2017-03-27 20:14:36 -04:00
parent 6462238f0d
commit a6f22e8356
4 changed files with 142 additions and 56 deletions
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29
doc/src/sgml/pgstatstatements.sgml
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@ -244,11 +244,12 @@
</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
<structfield>queryid</> hash value associated with the
When a constant's value has been ignored for purposes of matching the query
to other queries, the constant is replaced by a parameter symbol, such
as <literal>$1</literal>, in the <structname>pg_stat_statements</>
display.
The rest of the query text is that of the first query that had the
particular <structfield>queryid</> hash value associated with the
<structname>pg_stat_statements</> entry.
</para>
@ -301,6 +302,18 @@
replicas. If in doubt, direct testing is recommended.
</para>
<para>
The parameter symbols used to replace constants in
representative query texts start from the next number after the
highest <literal>$</><replaceable>n</> parameter in the original query
text, or <literal>$1</> if there was none. It's worth noting that in
some cases there may be hidden parameter symbols that affect this
numbering. For example, <application>PL/pgSQL</> uses hidden parameter
symbols to insert values of function local variables into queries, so that
a <application>PL/pgSQL</> statement like <literal>SELECT i + 1 INTO j</>
would have representative text like <literal>SELECT i + $2</>.
</para>
<para>
The representative query texts are kept in an external disk file, and do
not consume shared memory. Therefore, even very lengthy query texts can
@ -481,13 +494,13 @@ 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 + ? WHERE bid = ?;
query | UPDATE pgbench_branches SET bbalance = bbalance + $1 WHERE bid = $2;
calls | 3000
total_time | 9609.00100000002
rows | 2836
hit_percent | 99.9778970000200936
-[ RECORD 2 ]---------------------------------------------------------------------
query | UPDATE pgbench_tellers SET tbalance = tbalance + ? WHERE tid = ?;
query | UPDATE pgbench_tellers SET tbalance = tbalance + $1 WHERE tid = $2;
calls | 3000
total_time | 8015.156
rows | 2990
@ -499,7 +512,7 @@ total_time | 310.624
rows | 100000
hit_percent | 0.30395136778115501520
-[ RECORD 4 ]---------------------------------------------------------------------
query | UPDATE pgbench_accounts SET abalance = abalance + ? WHERE aid = ?;
query | UPDATE pgbench_accounts SET abalance = abalance + $1 WHERE aid = $2;
calls | 3000
total_time | 271.741999999997
rows | 3000