Re: 9.2.4 vs 9.3.0 query planning (sort merge join vs hash join)

[Jeff Janes <jeff.janes@xxxxxxxxx>]

On Mon, May 12, 2014 at 3:45 AM, Tim Kane <tim.kane@xxxxxxxxx> wrote:

Hi all,

Is there a propensity for 9.2.4 to prefer a sort-merge-join, in place of a hash join?

I’m fairly sure the answer is yes, but I also want to be sure I’m interpreting the explain output correctly.

I’m comparing behaviour between two systems, which for all intents and purposes are identical save for the version of postgres.

There appears to be nothing wrong with the row estimates given in the explain plan on either machine, however actual performance is significantly impaired on the 9.2.4 setup due to the preference for the use of a sort-merge join, compared to a hash-join on 9.3.0

Snippet from 9.2.4

                     ->  Merge Left Join  (cost=19598754.29..19602284.00 rows=469996 width=434) (actual time=6369152.750..6386029.191 rows=6866896 loops=1)

                           Buffers: shared hit=489837 read=1724585

                           ->  Sort  (cost=19598650.62..19599825.61 rows=469996 width=120) (actual time=6369151.307..6373591.881 rows=6866896 loops=1)                   (A)

                                 Sort Method: quicksort  Memory: 1162266kB

                                 Buffers: shared hit=489765 read=1724585

                                 ->  Hash Left Join  (cost=429808.90..19554371.62 rows=469996 width=120) (actual time=37306.534..6353455.046 rows=6866896 loops=1)        (B)

                                       Rows Removed by Filter: 20862464

                                       Buffers: shared hit=489765 read=1724585

Snippet from 9.3.0

                     ->  Hash Left Join  (cost=617050.43..20948535.43 rows=566893 width=434) (actual time=51816.864..934723.548 rows=6866896 loops=1)

                           Buffers: shared hit=1732 read=2010920 written=1

                           ->  Hash Left Join  (cost=616993.23..20870529.73 rows=566893 width=120) (actual time=51796.882..923196.579 rows=6866896 loops=1)

                                 Rows Removed by Filter: 20862464

                                 Buffers: shared hit=1732 read=2010877 written=1

As you can see, the estimates are similar enough between them, but 9.2.4 want’s to run sort-merge plan (A) – and the resulting execution time blows out hugely.

Intersetingly, it actually looks like it is the hash join immediately preceding the quick sort that isn’t performing well (B).   Though I suspect this is just how an explain plan reads - is this ultimately because the sort node is unable to retrieve tuples from the child node quickly enough?

It looks to me like a caching issue.  The two Hash Left Joins seem to be identical (although occurring on different levels of the plan) but one takes much more time.  But it is hard to know without seeing what if feeding into those hash joins.

Cheers,

Jeff

 

Setting enable_mergejoin = 0 appears to solve this, but I think an upgrade to 9.3.4 is going to win over.

Cheers,

Tim