[stats-dev] Attempting to speed movedata.sql for ogr-verification

Benjamin Gavin virtual_olympus at yahoo.com
Fri May 2 08:37:59 EDT 2003


OK,
  So I was looking through the queries that are part of the
ogr-verification process and trying to determine if we could speed any of
them up a bit.  As an example, I took the query which attempts to insert
non-matching stubs into the primary ogr_stubs table (i.e. those which came
in as returns, but don't currently exist in the ogr_stubs table).  I don't
have any real good performance numbers between the two, but initial tests
are showing that the new query is slightly faster than the old one:

Old Query:
SELECT DISTINCT L.stub_marks, 0
FROM logdata L
WHERE NOT EXISTS (SELECT * FROM ogr_stubs WHERE
			L.stub_marks = ogr_stubs.stub_marks);

Explain Output:
 Unique  (cost=3055.91..3059.66 rows=50 width=26)
   ->Sort  (cost=3055.91..3057.16 rows=500 width=26)
       Sort Key: stub_marks, 0
         ->Seq Scan on logdata l  (cost=0.00..3033.50 rows=500 width=26)
            Filter: (NOT (subplan))
            SubPlan
              ->Index Scan using stubs_marks on ogr_stubs
                (cost=0.00..3.01 rows=1 width=26)
                  Index Cond: ($0 = stub_marks)

New Query:
SELECT DISTINCT L.stub_marks, 0
FROM logdata L LEFT OUTER JOIN ogr_stubs O ON L.stub_marks = O.stub_marks
WHERE O.stub_marks IS NULL;

Explain Output:
 Unique  (cost=3095.83..3103.33 rows=100 width=46)
   ->Sort  (cost=3095.83..3098.33 rows=1000 width=46)
      Sort Key: l.stub_marks, 0
        ->Nested Loop  (cost=0.00..3046.00 rows=1000 width=46)
           Filter: ("inner".stub_marks IS NULL)
             ->Seq Scan on logdata l  (cost=0.00..20.00 rows=1000
width=26)
             ->Index Scan using stubs_marks on ogr_stubs o
               (cost=0.00..3.01 rows=1 width=20)
                  Index Cond: ("outer".stub_marks = o.stub_marks)


You'll notice that the cost estimate for the second query is higher, but
in my initial (admittedly minor) testing, the second tended to return as
much as 20% faster.  I'm fairly certain that the database load was rather
high at the time, and the numbers varied as much as 100% between runs, so
we'll need to do a bit more testing to see if we can eek out a bit more
performance from these.

Ben Gavin
thejet at distributed.net


=====
/******************************/
Benjamin Gavin
Virtual.Olympus Software
Virtual_Olympus at yahoo.com
http://www.virtual-olympus.com/
/******************************/


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