Change the sl_log_* selection query from using a complicated where clause
into a series of UNION ALL queries with slightly simpler qualifications.
The PostgreSQL optimizer failed on the old query to apply a lower bound
on the index scankey, causing the query to always select from the beginning
of the table. With a large backlog this caused significant time to be lost,
visible as "time to first row". The new query delivers the first log row
usually within milliseconds regardless of the size of sl_log_*.

New feature explain_interval.
This new slon.conf variable defines an interval in seconds at which the
remote worker will output the current log selection query together with
it's EXPLAIN query plan.

Branch
------
master

Details
-------
http://git.postgresql.org/gitweb?p=slony1-engine.git;a=commitdiff;h=d4118d06066db26d19a6acdc1d3580fceaf26b08

Modified Files
--------------
doc/adminguide/slonconf.sgml |   14 ++
share/slon.conf-sample       |    8 +-
src/slon/confoptions.c       |   13 ++
src/slon/remote_worker.c     |  469 +++++++++++++++++++++++++++---------------
src/slon/slon.h              |    1 +
5 files changed, 340 insertions(+), 165 deletions(-)