Re: limit 1 on view never finishes

On 10/27/16 3:46 PM, Craig James wrote:
>
> Limit (cost=3264.63..7193.14 rows=1 width=4)
> -> Nested Loop (cost=3264.63..428658697.57 rows=109114 width=4)
> Join Filter: (rv.version_id = sample.version_id)
> -> Index Only Scan Backward using version_pkey on version rv
> (cost=0.42..6812.85 rows=261895 width=4)
> -> Materialize (cost=3264.21..5992.06 rows=109114 width=4)
> -> HashAggregate (cost=3264.21..4355.35 rows=109114
> width=4)
> -> Seq Scan on sample (cost=0.00..2991.37
> rows=109137 width=4)
>
> Why would this trivial query run forever at 100% CPU?

My bet is that there's a lot of rows in version that have a higher
version than what's in sample. That means a lot of repeated scans
through the tuplestore underneath the Materialize node.

If you can remove duplicates from sample and get rid of the DISTINCT,
this will probably get a better plan. If you can't do that then you
could try changing the JOIN to an IN:

SELECT ... FROM sample
WHERE version_id IN (SELECT version_id FROM sample)
--
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
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