Re: Question about sorting internals
От | hubert depesz lubaczewski |
---|---|
Тема | Re: Question about sorting internals |
Дата | |
Msg-id | 20131211104110.GA5539@depesz.com обсуждение исходный текст |
Ответ на | Re: Question about sorting internals (Ashutosh Bapat <ashutosh.bapat@enterprisedb.com>) |
Список | pgsql-hackers |
On Wed, Dec 11, 2013 at 03:34:38PM +0530, Ashutosh Bapat wrote: > Hi deepesz, > You might want to see their EXPLAIN VERBOSE outputs. Having one of them > (2004 one) lesser number of rows, might be getting picked up as first > relation being union and thus ends up having it's rows before the second > one. Explain output would make it more clear. Also, try having same number > of rows in both the relations. Explains: QUERY PLAN --------------------------------------------------------------------------------------------------------------------Unique (cost=0.44..0.48 rows=9 width=12) (actual time=0.030..0.035 rows=6 loops=1) Output: rok2004.miesiac, (2004), rok2004.wynik CTE rok2004 -> Values Scan on "*VALUES*" (cost=0.00..0.06 rows=5 width=8) (actual time=0.001..0.003 rows=5loops=1) Output: "*VALUES*".column1, "*VALUES*".column2 CTE rok2005 -> Values Scan on "*VALUES*_1" (cost=0.00..0.05rows=4 width=8) (actual time=0.000..0.001 rows=4 loops=1) Output: "*VALUES*_1".column1, "*VALUES*_1".column2 -> Sort (cost=0.32..0.35 rows=9 width=12) (actual time=0.029..0.031 rows=9 loops=1) Output:rok2004.miesiac, (2004), rok2004.wynik Sort Key: rok2004.miesiac, rok2004.wynik Sort Method: quicksort Memory: 25kB -> Append (cost=0.00..0.18 rows=9 width=12) (actual time=0.007..0.018 rows=9 loops=1) -> CTE Scan on rok2004 (cost=0.00..0.10 rows=5 width=12) (actual time=0.006..0.011 rows=5 loops=1) Output: rok2004.miesiac, 2004, rok2004.wynik -> CTE Scan on rok2005 (cost=0.00..0.08 rows=4 width=12)(actual time=0.002..0.004 rows=4 loops=1) Output: rok2005.miesiac, 2005, rok2005.wynikTotal runtime:0.077 ms (18 rows) QUERY PLAN --------------------------------------------------------------------------------------------------------------------Unique (cost=0.44..0.48 rows=9 width=12) (actual time=0.024..0.027 rows=6 loops=1) Output: rok2005.miesiac, (2005), rok2005.wynik CTE rok2004 -> Values Scan on "*VALUES*" (cost=0.00..0.06 rows=5 width=8) (actual time=0.001..0.003 rows=5loops=1) Output: "*VALUES*".column1, "*VALUES*".column2 CTE rok2005 -> Values Scan on "*VALUES*_1" (cost=0.00..0.05rows=4 width=8) (actual time=0.001..0.003 rows=4 loops=1) Output: "*VALUES*_1".column1, "*VALUES*_1".column2 -> Sort (cost=0.32..0.35 rows=9 width=12) (actual time=0.023..0.024 rows=9 loops=1) Output:rok2005.miesiac, (2005), rok2005.wynik Sort Key: rok2005.miesiac, rok2005.wynik Sort Method: quicksort Memory: 25kB -> Append (cost=0.00..0.18 rows=9 width=12) (actual time=0.004..0.015 rows=9 loops=1) -> CTE Scan on rok2005 (cost=0.00..0.08 rows=4 width=12) (actual time=0.003..0.006 rows=4 loops=1) Output: rok2005.miesiac, 2005, rok2005.wynik -> CTE Scan on rok2004 (cost=0.00..0.10 rows=5 width=12)(actual time=0.001..0.006 rows=5 loops=1) Output: rok2004.miesiac, 2004, rok2004.wynikTotal runtime:0.053 ms (18 rows) So, it looks like rowcount is the one thing that's different. Not entirely sure how the logic would be to make rowcount differ. After some more talk on #postgresql, it looks like I will have to spend some time with debugger to see what's happening there. Best regards, depesz
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