Re: how to efficiently update tuple in many-to-many relationship?

On 4/9/07, Drew Wilson <drewmwilson(at)gmail(dot)com> wrote:
> I have 2 tables (A,B) joined in a many-to-many relationship via a
> join table ("membership"), where updating table A based on table B
> takes a very long time.
>
> Tables A and B have oid primary keys (a_id and b_id respectively).
> The join table, "membership", has its own oid primary key
> (membership_id), as well as foreign keys "a_id" and "b_id".
>
> A SELECT query across all 3 tables takes 12 seconds.
> "SELECT count(*) FROM a JOIN membership USING(a_id) JOIN b USING
> (b_id) WHERE b.is_public = true"
>
> But a simple UPDATE using the same SELECT query takes 30 minutes to
> an hour.
> "UPDATE A set is_public=true WHERE a_id IN (SELECT count(*) FROM a
> JOIN membership USING(a_id) JOIN b USING(b_id) WHERE b.is_public =
> true)".
>
> What am I doing wrong here? I'm not sure how to diagnose this further.
>
> Here's the output from explain:
> db=# EXPLAIN SELECT a_id FROM a JOIN membership USING(a_id) JOIN b
> USING(b_id) WHERE b.is_public = true;
> ------------------------------------------------------------------------
> -----------------------------------------
> Hash Join (cost=167154.78..173749.48 rows=51345 width=4)
> Hash Cond: (a.a_id = membership.a_id)
> -> Function Scan on a (cost=0.00..12.50 rows=1000 width=4)
> -> Hash (cost=144406.48..144406.48 rows=1819864 width=4)
> -> Hash Join (cost=417.91..144406.48 rows=1819864 width=4)
> Hash Cond: (membership.b_id = b.b_id)
> -> Seq Scan on membership (cost=0.00..83623.83
> rows=4818983 width=8)
> -> Hash (cost=348.52..348.52 rows=5551 width=4)
> -> Index Scan using b_is_public on b
> (cost=0.00..348.52 rows=5551 width=4)
> Index Cond: (is_public = true)
> Filter: is_public
> (11 rows)
>
>
> db=# EXPLAIN UPDATE a SET is_public = true WHERE a_id IN
> ( SELECT a_id FROM a JOIN membership USING(a_id) JOIN b USING
> (b_id) WHERE b.is_public = true);
> ------------------------------------------------------------------------
> -----------------------------------------
> hash in join (cost=281680.17..370835.63 rows=1819864 width=90)
> hash cond: (public.a.a_id = public.a.a_id)
> -> seq scan on a (cost=0.00..47362.09 rows=2097309 width=90)
> -> hash (cost=258931.87..258931.87 rows=1819864 width=8)
> -> hash join (cost=73996.36..258931.87 rows=1819864 width=8)
> hash cond: (membership.a_id = public.a.a_id)
> -> hash join (cost=417.91..144406.48 rows=1819864
> width=4)
> hash cond: (membership.b_id = b.b_id)
> -> seq scan on membership
> (cost=0.00..83623.83 rows=4818983 width=8)
> -> hash (cost=348.52..348.52 rows=5551 width=4)
> -> index scan using
> loc_submission_is_public on b (cost=0.00..348.52 rows=5551 width=4)
> index cond: (is_public = true)
> filter: is_public
> -> hash (cost=47362.09..47362.09 rows=2097309 width=4)
> -> seq scan on a (cost=0.00..47362.09
> rows=2097309 width=4)

why don't you rewrite your update statement to use joins (joins >
where exists > where in)?

WHERE a_id IN (SELECT count(*) FROM a
the above looks wrong maybe?

merlin