Gregory Stark wrote:
tmp <[EMAIL PROTECTED]> writes:
If the input is "sufficiently" large and the LIMIT threshold "sufficiently"
small, maintain the DISTINCT output by hashning while traversing the input and
stop when the LIMIT threshold is reached. No sorting required and *at* *most*
one read of input.
You mean like this?
postgres=# explain select distinct x from i limit 5;
QUERY PLAN
-------------------------------------------------------------------
Limit (cost=54.50..54.51 rows=1 width=304)
-> HashAggregate (cost=54.50..54.51 rows=1 width=304)
-> Seq Scan on i (cost=0.00..52.00 rows=1000 width=304)
(3 rows)
Does that know to stop scanning as soon as it has seen 5 distinct values?
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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