Question

Immediate disclaimer: Its been a few years since I seriously did SQL, returning recently.

Given a table:

logical_id, instance_id, node_id, state, time_stamp
1, 1, a, starting, 1300003234
2, 1, a, up, 1300003235
**3, 1, b, starting, 1300003237**
4, 1, a, completed, 1300003239
5, 1, a, shutdown, 1300003240

Given a specific instance_id (e.g. 1) I want to get the last state record (single row- highlighted in bold) of the authoritive node, the authoritive node is the most recent one to have entered a ‘starting’, in the absence of any node entering a starting stage it just the most recent node to be logged.

I figured I do some form group by node_id with order by (rather than filter) boolean(state = ‘starting), time_stamp desc, but I cant seem to get this right, in addition I think the group by would have to come after the ordering.

Im using postgres 9.x

Answer 1

This will retrieve the most recent stating node, if it exists, otherwise it will simply return the most recent non-starting node.

SELECT U.logical_id, U.instance_id, U.node_id, U.state, U.time_stamp
FROM 
(
 SELECT M1.logical_id, M1.instance_id, M1.node_id, M1.state, M1.time_stamp, 1 AS node_type
 FROM MyTable M1
 WHERE M1.instance_id=1 AND M1.state='starting' 
 ORDER BY M1.time_stamp DESC LIMIT 1

 UNION

 SELECT M2.logical_id, M2.instance_id, M2.node_id, M2.state, M2.time_stamp, 0 AS node_type
 FROM MyTable M2
 WHERE M2.instance_id=1 
 ORDER BY M2.time_stamp DESC LIMIT 1
) AS U
ORDER BY U.node_type DESC  LIMIT 1