Warum nimmt LIMIT 2 in dieser Abfrage Größenordnungen länger als LIMIT 1?

Question

Ich bin auf PostgreSQL 9.3. Dies sollte auf jedem Tisch mit mehr als 100.000 Zeilen reproduzieren. Die EXPLAIN ANALYZE zeigt viel mehr Zeilen, die mit LIMIT 2 gescannt wurden, aber ich kann nicht herausfinden warum.

Limit 1:

EXPLAIN ANALYZE WITH base AS (
    SELECT *, ROW_NUMBER() OVER() AS rownum FROM a_big_table 
), filter AS (
    SELECT rownum, true AS thing FROM base 
) SELECT * FROM base LEFT JOIN filter USING (rownum) WHERE filter.thing LIMIT 1 

Ergebnis:

Limit (cost=283512.19..283517.66 rows=1 width=2114) (actual time=0.019..0.019 rows=1 loops=1) 
    CTE base 
    -> WindowAgg (cost=0.00..188702.69 rows=4740475 width=101) (actual time=0.008..0.008 rows=1 loops=1) 
      -> Seq Scan on a_big_table (cost=0.00..129446.75 rows=4740475 width=101) (actual time=0.003..0.003 rows=1 loops=1) 
    CTE filter 
    -> CTE Scan on base base_1 (cost=0.00..94809.50 rows=4740475 width=8) (actual time=0.000..0.000 rows=1 loops=1) 
    -> Nested Loop (cost=0.00..307677626611.24 rows=56180269915 width=2114) (actual time=0.018..0.018 rows=1 loops=1) 
     Join Filter: (base.rownum = filter.rownum) 
     -> CTE Scan on base (cost=0.00..94809.50 rows=4740475 width=2113) (actual time=0.011..0.011 rows=1 loops=1) 
     -> CTE Scan on filter (cost=0.00..94809.50 rows=2370238 width=9) (actual time=0.002..0.002 rows=1 loops=1) 
       Filter: thing 
Total runtime: 0.057 ms 

Limit 2:

EXPLAIN ANALYZE WITH base AS (
    SELECT *, ROW_NUMBER() OVER() AS rownum FROM a_big_table 
), filter AS (
    SELECT rownum, true AS thing FROM base 
) SELECT * FROM base LEFT JOIN filter USING (rownum) WHERE filter.thing LIMIT 2 

Ergebnis:

Limit (cost=283512.19..283523.14 rows=2 width=2114) (actual time=0.018..14162.283 rows=2 loops=1) 
    CTE base 
    -> WindowAgg (cost=0.00..188702.69 rows=4740475 width=101) (actual time=0.008..4443.359 rows=4714243 loops=1) 
      -> Seq Scan on a_big_table (cost=0.00..129446.75 rows=4740475 width=101) (actual time=0.002..1421.622 rows=4714243 loops=1) 
    CTE filter 
    -> CTE Scan on base base_1 (cost=0.00..94809.50 rows=4740475 width=8) (actual time=0.001..10214.684 rows=4714243 loops=1) 
    -> Nested Loop (cost=0.00..307677626611.24 rows=56180269915 width=2114) (actual time=0.018..14162.280 rows=2 loops=1) 
     Join Filter: (base.rownum = filter.rownum) 
     Rows Removed by Join Filter: 4714243 
     -> CTE Scan on base (cost=0.00..94809.50 rows=4740475 width=2113) (actual time=0.011..0.028 rows=2 loops=1) 
     -> CTE Scan on filter (cost=0.00..94809.50 rows=2370238 width=9) (actual time=0.009..6595.770 rows=2357122 loops=2) 
       Filter: thing 
Total runtime: 14247.374 ms 

Answer 1

Motor läuft zuerst, dann begrenzt. Sie können also viel mehr Zeilen sehen.