On 01/12/2015 11:33 PM, Petr Jelinek wrote:
Second patch adds DDL support. I originally wanted to make it CREATE/DROP SEQUENCE ACCESS METHOD... but that would mean making ACCESS a reserver keyword so I went for CREATE ACCESS METHOD FOR SEQUENCES which does not need to change anything (besides adding METHOD to unreserved keywords). The DDL support uses the DefineStmt infra with some very small change as the sequence ams are not schema qualified, but I think it's acceptable and saves considerable amount of boilerplate.
Do we need DDL commands for this at all? I could go either way on that question. We recently had a discussion on that wrt. index access methods [1], and Tom opined that providing DDL for creating index access methods is not worth it. The extension can just insert the rows into pg_seqam with INSERT. Do we expect sequence access methods as extensions to be more popular than index access methods? Maybe, because the WAL-logging problem doesn't exist. But OTOH, if you're writing something like a replication system that needs global sequences as part of it, there aren't that many of those, and the installation scripts will need to deal with more complicated stuff than inserting a row in pg_seqam.
[1] http://www.postgresql.org/message-id/26822.1414516...@sss.pgh.pa.us
And third patch is gapless sequence implementation updated to work with the new DDL support with some tests added for checking if dependencies work correctly. It also acts as example on how to make custom AMs.
I'll take a look at that to see how the API works, but we're not going to include it in the source tree, because it doesn't actually guarantee gaplessness. That makes it a pretty dangerous example. I'm sure we can come up with a better example that might even be useful. How about a Lamport's clock sequence, which advances once per second, in addition to when anyone calls nextval() ? Or a remote sequence that uses an FDW to call nextval() in a foreign server?
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