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
alban@xxxxxxxxxxxxxxxxx
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