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A sharp drop in performance SELECT .. WHERE NOT IN (SELECT ..) with strings

PROBLEM:

SELECT new_filename
FROM tmp2_import_lightnings_filenames
WHERE new_filename
    NOT IN (SELECT filename FROM service.import_lightnings_filenames LIMIT 64500)
LIMIT 1;

Runtime: 62 ms .

SELECT new_filename
FROM tmp2_import_lightnings_filenames
WHERE new_filename
    NOT IN (SELECT filename FROM service.import_lightnings_filenames LIMIT 65000)
LIMIT 1;

Runtime: 4.742 s .

(All LIMITS are for testing purposes only).

A huge lag! And it is increasing exponentially.

TABLES:

CREATE TABLE public.tmp2_import_lightnings_filenames (
  new_filename VARCHAR(63) NOT NULL, 
  CONSTRAINT tmp2_import_lightnings_filenames_pkey PRIMARY KEY(new_filename)
) WITHOUT OIDS;

Table Size: 7304 Rows

Sample data: /xml/2012-07-13/01/01-24.xml

CREATE TABLE service.import_lightnings_filenames (
  id SERIAL, 
  filename VARCHAR(63) NOT NULL, 
  imported BOOLEAN DEFAULT false, 
  strokes_num INTEGER, 
  CONSTRAINT import_lightnings_filenames_pkey PRIMARY KEY(id)
) WITHOUT OIDS;

CREATE UNIQUE INDEX import_lightnings_filenames_idx
ON service.import_lightnings_filenames
USING btree (filename COLLATE pg_catalog."default");

Table Size: Rows 70812

Sample data: 44;/xml/2012-05-26/12/12-18.xml;TRUE;NULL

QUERY REQUESTS:

 Limit  (cost=0.00..2108.11 rows=1 width=29) (actual time=240.183..240.183 rows=1 loops=1)
   Buffers: shared hit=539, temp written=307
   ->  Seq Scan on tmp2_import_lightnings_filenames  (cost=0.00..7698823.12 rows=3652 width=29) (actual time=240.181..240.181 rows=1 loops=1)
         Filter: (NOT (SubPlan 1))
         Buffers: shared hit=539, temp written=307
         SubPlan 1
           ->  Materialize  (cost=0.00..1946.82 rows=64500 width=29) (actual time=0.009..198.313 rows=64500 loops=1)
                 Buffers: shared hit=538, temp written=307
                 ->  Limit  (cost=0.00..1183.32 rows=64500 width=29) (actual time=0.005..113.196 rows=64500 loops=1)
                       Buffers: shared hit=538
                       ->  Seq Scan on import_lightnings_filenames  (cost=0.00..1299.12 rows=70812 width=29) (actual time=0.004..42.418 rows=64500 loops=1)
                             Buffers: shared hit=538
 Total runtime: 240.982 ms



  Limit  (cost=0.00..2125.03 rows=1 width=29) (actual time=30734.619..30734.619 rows=1 loops=1)
   Buffers: shared hit=547, temp read=112258 written=669
   ->  Seq Scan on tmp2_import_lightnings_filenames  (cost=0.00..7760626.00 rows=3652 width=29) (actual time=30734.617..30734.617 rows=1 loops=1)
         Filter: (NOT (SubPlan 1))
         Buffers: shared hit=547, temp read=112258 written=669
         SubPlan 1
           ->  Materialize  (cost=0.00..1962.49 rows=65000 width=29) (actual time=0.798..42.306 rows=64820 loops=363)
                 Buffers: shared hit=543, temp read=112258 written=669
                 ->  Limit  (cost=0.00..1192.49 rows=65000 width=29) (actual time=0.005..116.110 rows=65000 loops=1)
                       Buffers: shared hit=543
                       ->  Seq Scan on import_lightnings_filenames  (cost=0.00..1299.12 rows=70812 width=29) (actual time=0.003..43.804 rows=65000 loops=1)
                             Buffers: shared hit=543
 Total runtime: 30735.267 ms

What am I doing wrong?

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2 answers

The reason for the performance drop is that you are exhausted and the step begins to replace with a disk. I quote the instruction here: work_memmaterialize

work_mem ( )
[...] - -, - < IN.

. , work_mem . @a_horse , , :

set work_mem = '64MB';

. reset :

reset work_mem;

. postgresql.conf ( ) .

PostgreSQL ( 1 ). , 16 4 . 64 db 12 - .

, . Wiki PostgreSQL. work_mem .

, , , . IN , , PostgreSQL.

LEFT JOIN/IS NULL

SELECT new_filename
FROM   tmp2_import_lightnings_filenames t
LEFT   JOIN (
    SELECT filename
    FROM   service.import_lightnings_filenames
    LIMIT  65000
    ) x ON t.new_filename = x.filename 
WHERE  x.filename IS NULL;

, , service.import_lightnings_filenames:

SELECT new_filename
FROM   tmp2_import_lightnings_filenames t
WHERE  NOT EXISTS (
    SELECT 1
    FROM (
        SELECT filename
        FROM   service.import_lightnings_filenames
        LIMIT  65000
        ) x
    WHERE t.new_filename = x.filename 
    );

CTE (, , ):

WITH x AS (
    SELECT filename
    FROM   service.import_lightnings_filenames
    LIMIT  65000
    )
SELECT new_filename
FROM   tmp2_import_lightnings_filenames t
WHERE  NOT EXISTS (
    SELECT 1
    FROM   x
    WHERE  t.new_filename = x.filename 
    );
+3
-- SET work_mem=20000;
SET random_page_cost=1.1;
SET effective_cache_size=10000000;

work_mem 1--20 - ( ). .

random_page_cost , . OP. ( , , seqscan) (= 4) )

effective_cache_size - LRU, . ( )

0

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