Possible fails in pg_stat_statements test

Hello,

There are some places in the pg_state_statement's regress test where the 
bool result of comparison between the number of rows obtained and 
wal_records generated by query should be displayed.
Now counting the number of wal_records for some query in 
pg_state_statement is done by the global pgWalUsage.wal_records counter 
difference calculation.
During query execution the extra wal_records may appear that are not 
relate to the query.
There are two reasons why this might happen:
1) Owing to the hit into pruning of some page in optional pruning 
function (heap_page_prune_opt()).
2) When a new page is required for a new xid in clog and 
WriteZeroPageXlogRec() was called.
In both cases an extra wal record with zero xl_xid is generated, so 
wal_records counter gives an incremented value for this query and 
pg_stat_statement test will fail.

This patch introduces an additional counter of wal records not related 
to the query being executed.
Due to this counter pg_stat_statement finds out the number of wal 
records that are not relevant to the query and does not include them in 
the per query statistic.
This removes the possibility of the error described above.

There is a way to reproduce this error when patch is not applied:
1) start server with "shared_preload_libraries = 'pg_stat_statements'" 
string in the postgresql.conf;
2) replace makefile in contrib/pg_stat_statements with attached one;
3) replace test file 
contrib/pg_stat_statements/sql/pg_stat_statements.sql and expected 
results contrib/pg_stat_statements/expected/pg_stat_statements.out
with shorter versions from attached files;
4) copy test.sh to contrib/pg_stat_statements and make sure that PGHOME 
point to your server;
5) cd to contrib/pg_stat_statements and execute:
export ITER=1 && while ./start.sh || break; export ITER=$(($ITER+1)); do 
:; done

Usually 100-200 iterations will be enough.
To catch the error more faster one can add wal_records column to SELECT
in line 26 of contrib/pg_stat_statements/sql/pg_stat_statements.sql as 
followes:
SELECT query, calls, rows, wal_records,
and replace the contrib/pg_stat_statements/expected/pg_stat_statements.out
with attached pg_stat_statements-fast.out

With best regards,
-- 
Anton A. Melnikov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company