Re: Perfomance of IN-clause with many elements and possible solutions

Dmitry Lazurkin <dilaz03@xxxxxxxxx> · Tue, 25 Jul 2017 01:22:30 +0300



    On 25.07.2017 01:15, David G. Johnston wrote:

    
        On Mon, Jul 24, 2017 at
            3:12 PM, Dmitry Lazurkin <dilaz03@xxxxxxxxx> wrote:

        
              And I have one
                question. I don't understand why IN-VALUES doesn't use
                Semi-Join? PostgreSQL has Hash Semi-Join...  For which
                task the database has node of this type?

              
            Semi-Join
              is canonically written as:
            

            SELECT *
            FROM tbl
            WHERE
              EXISTS (SELECT 1 FROM tbl2 WHERE tbl.id = tbl2.id)
            

            The main
              difference between IN and EXISTS is NULL semantics.
            

            David J.
            

    ALTER TABLE ids ALTER COLUMN id SET NOT NULL;

    EXPLAIN (ANALYZE, BUFFERS) SELECT count(*) FROM ids WHERE id IN
    :values_clause; 

    
     Aggregate  (cost=245006.46..245006.47 rows=1 width=8) (actual
    time=3824.095..3824.095 rows=1 loops=1)

       Buffers: shared hit=44248

       ->  Hash Join  (cost=7.50..235006.42 rows=4000019 width=0)
    (actual time=1.108..3327.112 rows=3998646 loops=1)

       ...

    
    Hmmm. No Semi-Join.

    
    PostgreSQL can use Semi-Join for IN too.