Re: Potential optimisation for the creation of a partial indexcondition over a newly created nullable, non-default-valued column? - Mailing list pgsql-general
From
Tim Dawborn
Subject
Re: Potential optimisation for the creation of a partial indexcondition over a newly created nullable, non-default-valued column?
While it could be technically possible to do something like check the xmin of the pg_attribute record for all columns mentioned in the index's predicate all are set to the current transaction ID and the index predicate refutes an expression containing those columns with IS NULL clauses or whatever the DEFAULT expression value is, we've just no way to know if any rows were inserted or updated between the ALTER TABLE and the CREATE INDEX. Certainly, no other transaction could have done anything since we added the column due to us holding the AccessExclusiveLock. We just don't really have a way to know if our own transaction did anything in between. So to do this, we'd need to invent something to track that, and that something wouldn't be free to maintain and we'd likely need to maintain it all the time since we'd be unable to predict what future commands might need to use that information.
I see. Thanks for explaining. I'm not familiar with the internals of Postgres. I thought there might have already been something akin to a "dirty" bit for each table for each transaction to indicate whether or not it had been modified in some way, which could have been used for this hypothetical process. It sounds like that's not the case, in which case yes, the additional overhead of this bookkeeping required for this feature would be unjustifiable.
It would likely be easier to go about it by having some sort of ALTER TABLE ADD INDEX command, then just chain the alter tables together. We have various other optimisations when multiple subcommands are used in a single ALTER TABLE. However, I'm not suggesting we go and allow indexes to be created in ALTER TABLE. I"m just suggesting that it would likely be a better alternative than inventing something to track when a table last had
Nice. I hadn't considered that as a way to go about solving this problem. Having a quick search online for ALTER TABLE CREATE INDEX syntax, it appears that MySQL does support this notion (via ALTER TABLE ADD INDEX), though it doesn't (currently) support partial indexes. Having a quick look in the mailing list archives, I couldn't find any discussions about supporting this syntax. What are the arguments against adding such additional syntax (other than the standard issues that come with adding additional syntax)?
