pg_stat_statements fwrite not save
背景
PostgreSQL 9.4 对pg_stat_statements插件的一个重大修改，它在使用过程中需要使用文件，所有的操作都会fwrite文件。
http://www.postgresql.org/docs/9.4/static/release-9-4.html
这么做的好处是，可以记录很多SQL。
但是这里也带来了其他问题，IO的问题，当然WRITE操作会由操作系统调度刷脏页，所以实际上可能不会产生太多的文件系统IO。
不过即使你pg_stat_statements.save=off，也会写文件，记住了哦。
因为save=off只是在启动和关闭时清理文件，而在使用过程中都是需要写文件的。
E.5.3.12.2. pg_stat_statements
Make pg_stat_statements use a file, rather than shared memory, for query text storage (Peter Geoghegan)
This removes the previous limitation on query text length, and allows a higher number of unique statements to be tracked by default.
Allow reporting of pg_stat_statements's internal query hash identifier (Daniel Farina, Sameer Thakur, Peter Geoghegan)
Add the ability to retrieve all pg_stat_statements information except the query text (Peter Geoghegan)
This allows monitoring tools to fetch query text only for just-created entries, improving performance during repeated querying of the statistics.
Make pg_stat_statements ignore DEALLOCATE commands (Fabien Coelho)
It already ignored PREPARE, as well as planning time in general, so this seems more consistent.
Save the statistics file into $PGDATA/pg_stat at server shutdown, rather than $PGDATA/global (Fujii Masao)
详见代码：
contrib/pg_stat_statements/pg_stat_statements.c
这三个函数会
pgss_post_parse_analyze
pgss_ExecutorEnd
pgss_ProcessUtility
调用
pgss_store
调用
qtext_store
调用
write 写文件
9.3 以及以前的版本，如果pg_stat_statements.save=off, 则绝对不会写文件。只有=on时会把内存中的信息dump到pg_stat/*中。