On Thu, May 18, 2017 at 1:18 PM, Rob Brucks <rob.bru...@rackspace.com> wrote:
> According to this post, adding "if not exists" won't really help for race > conditions. > > > > "The bottom line is that CREATE TABLE IF NOT EXISTS doesn't pretend to > > handle concurrency issues any better than regular old CREATE TABLE, > > which is to say not very well." - Robert Haas > > > > https://www.postgresql.org/message-id/CA+TgmoZAdYVtwBfp1FL2sMZbiHCWT4UP > rzrlnnx1nb30ku3...@mail.gmail.com > > > > It still doesn't explain how the function got past creating the table, but > failed on the index. If another thread was also creating the table then > there should have been lock contention on the create table statement. > > > AT1: Insert, failed, cannot find table AT2: Insert, failed, cannot find table BT2: Create Table, succeeds BT1: Create Table; fails, it exists now, if exists converts to a warning CT2: Create Index, succeeds CT1: Create Index, fails , hard error DT2: Insert, succeeds DT1: Never Happens What that post seems to be describing is that it is possible the "BT1" actually hard errors instead of just being converted into a notice. There is no statement visible action to show that interleave but there is an underlying race condition since both BT1 and BT2 are executing concurrently. In short even with IF NOT EXISTS you are not guaranteed to not fail. But at least IF NOT EXISTS makes the probability of not failing > 0. It doesn't handle the concurrency any better - but it does change the outcome in some of those less-than-ideally handled situations. David J.
