> I'm thinking I should try to move all these decisions to phase 1 as > much as possible but I'm not sure how feasible it will be to get the > results exactly correct. Of course the cases where it's hardest to > predict are precisely where users would most like to know what's going > to happen...
Maybe you can move some of these decisions to phase 1, but I'm not sure it can be done for all of them. Another possible plan is to add a flag "dry run" so that phases 2/3 do whatever analysis they need to report for your EXPLAIN, but not actually carry out their tasks. (I see two options to implement this, one is a global flag and the other is a new argument to all those routines.)
You need to take a table lock to find out things about the table.
EXPLAIN seems like the wrong place for this.
I suggest ALTER TABLE should respond to a parameter setting of ddl_dry_run = on, so the whole world doesn't need to rewrite its syntax to support the new option.
--
Simon Riggs http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
