Collecting executed queries
===========================
This PostgreSQL extension allows you to collect all executed queries
into a file. So that you can analyze or replay them later.

Most of the code that interacts directly with the executor comes from
the auto_explain and pg_stat_statements extensions (hooks, shared
memory management etc).


Install
-------
Installing the extension is quite simple, especially if you're on 9.1.
In that case all you need to do is this:

   $ make install

and the (after connecting to the database)

   db=# CREATE EXTENSION query_recorder;

If you're on pre-9.1 version, you'll have to do the second part manually
by running the SQL script (query_recorder--x.y.sql) in the database. If
needed, replace MODULE_PATHNAME by $libdir.


Config
------
Now the extension is installed, but you still need to load the shared
module. This needs to be done from postgresql.conf, as the module
needs to allocate space in the shared memory segment. So add this to
the config file (or update the current values)

   # libraries to load
   shared_preload_libraries = 'query_recorder'

   # known GUC prefixes
   custom_variable_classes = 'query_recorder'

   # config of the query histogram
   query_recorder.filename = '/tmp/queries.log'
   query_recorder.max_files = 100
   query_recorder.size_limit = 1GB
   query_recorder.buffer_size = 8MB
   query_recorder.enabled = false

The meaning of those config options is this:

  (a) query_recorder.filename - filename where to store the recorded
      queries (the file number will be appended to the filename)

  (b) query_recorder.max_files - number of files to rotate (the number
      will be appended to the filename)

  (c) query_recorder.size_limit - size limit for each file

  (d) query_recorder.buffer_size - size of the buffer used to store the
      queries before writing them to the file

  (e) query_recorder.enabled - is the recording enabled or disabled

By default, the recording is disabled - you have to enable it, either
in postgresql.conf or by setting it later

   SET query_recorder.enabled = true

and later disable it by

   SET query_recorder.enabled = false

So you may enable/disable the query recording as needed.

The queries will be written to the file, and every time the file reaches
the size limit, a new file (with an incremented sequence number) will be
created. So for example if you set the filename like this

   query_recorder.filename = '/tmp/queries.log'

then the first file will be '/tmp/queries.log.000', when it reaches the
size limit a file '/tmp/queries.log.001' will be created etc. up to 
'/tmp/queries.log.999' (or whatever limit you've set using max_files).

When the last file is filled, the first one (.000) is recycled etc.


Reading the file
----------------

The file is quite simple to read and process. There are five fields

  - timestamp (with microseconds)
  - ID of the backend
  - duration of the query
  - length of the query (number of characters)
  - query (may include EOL, use the previous field to parse it)