[pgpool-hackers: 4080] Proposal: allow to specify replication delay by time

[Tatsuo Ishii]

Currently delay_threshold specifies replication delay upper limit in
bytes. It will be more intuitive to specify the replication delay by
time like '10 seconds'. For this purpose, we can use
(pg_last_committed_xact()).timestamp instead of pg_current_wal_lsn(),
and pg_last_xact_replay_timestamp() instead of
pg_last_wal_replay_lsn(). One thing we need to care about is, to use
pg_last_committed_xact(), track_commit_timestamp (available in
PostgreSQL 9.5 or after) must be enabled. If this is not enabled,
pg_last_committed_xact() raises an error. Also the function returns
NULL if no transaction is committed since the system started.

Comments and/or suggestions are welcome.
--
Tatsuo Ishii
SRA OSS, Inc. Japan
English: http://www.sraoss.co.jp/index_en.php
Japanese:http://www.sraoss.co.jp