Martijn van Oosterhout wrote:
> ISTM that what would really work well is some kind of "Merge Sort" node
> that would work by having multiple subnodes which are already sorted
> and merging them into one sorted list.
Would... So this isn't available yet?
> The planner would use this whenever it saw a query of the form:
>
> SELECT * FROM a
> UNION ALL
> SELECT * FROM b
> ORDER BY c;
>
> It would push the ORDER BY down to the subqueries and then merge the
> results. If the subqueries can be read efficiently sorted (via an index
> for example) then you would get very quick output, especially if you
> have a LIMIT clause.
I just realized that OFFSET kind of complicates the problem.
If PostgreSQL would handle this (for inheritance as well, I hope), it'd
need to keep track of how many records came from which tables to set the
offsets in the subqueries appropriately, which of course depends on the
previous query... Well, I said it complicates things...
--
Alban Hertroys
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