Reduce lock levels for ADD and DROP COLUMN

Idea is to reduce lock level of ADD/DROP COLUMN from AccessExclusiveLock
down to ShareRowExclusiveLock.

To make it work, we need to recognise that we are adding a column
without rewriting the table. That's a simple test at post parse analysis
stage, but what I can't do is work out whether the column name used is a
domain name which contains a constraint.

So if we want this, then it seems we need to add a separate subcommand,
so we can then throw an error if a domain is specified.

ALTER TABLE foo
  ADD [COLUMN] colname CONCURRENTLY;

Or other ideas? Do we really care?

DROP ... RESTRICT works fine at reduced lock level, assuming I'm not
missing anything...

ALTER TABLE foo
  DROP [COLUMN] colname RESTRICT;

Patch needs docs, tests and a check for the domain, so just a quick hack
just to get my dev muscles back in shape after Christmas. (Jokes
please).

Comments?

-- 
 Simon Riggs           http://www.2ndQuadrant.com/books/
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