On 09/30/2014 02:51 PM, Kevin Grittner wrote: > Josh Berkus <j...@agliodbs.com> wrote: >> On 09/30/2014 02:39 PM, Kevin Grittner wrote: >>> Josh Berkus <j...@agliodbs.com> wrote: >>>> On 09/30/2014 07:15 AM, Kevin Grittner wrote: >>>> >>>>> At the risk of pushing people away from this POV, I'll point out >>>>> that this is somewhat similar to what we do for unlogged bulk loads >>>>> -- if all the conditions for doing it the fast way are present, we >>>>> do it the fast way; otherwise it still works, but slower. >>>> >>>> Except that switching between fast/slow bulk loads affects *only* the >>>> speed of loading, not the locking rules. Having a statement silently >>>> take a full table lock when we were expecting it to be concurrent >>>> (because, for example, the index got rebuilt and someone forgot the >>>> UNIQUE) violates POLA from my perspective. >>> >>> I would not think that an approach which took a full table lock to >>> implement the more general case would be accepted. >> >> Why not? There are certainly cases ... like bulk loading ... where >> users would find it completely acceptable. Imagine that you're merging >> 3 files into a single unlogged table before processing them into >> finished data. > > So the expectation is that when we implement MERGE it will, by > default, take out an EXCLUSIVE lock for the entire target table for > the entire duration of the command? I would have expected a bit > more finesse.
I don't know that that is the *expectation*. However, I personally would find it *acceptable* if it meant that we could get efficient merge semantics on other aspects of the syntax, since my primary use for MERGE is bulk loading. Regardless, I don't think there's any theoretical way to support UPSERT without a unique constraint. Therefore eventual support of this would require a full table lock. Therefore having it use the same command as UPSERT with a unique constraint is a bit of a booby trap for users. This is a lot like the "ADD COLUMN with a default rewrites the whole table" booby trap which hundreds of our users complain about every month. We don't want to add more such unexpected consequences for users. -- Josh Berkus PostgreSQL Experts Inc. http://pgexperts.com -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers