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Re: LIMIT causes planner to do Index Scan using a less optimal index - Mailing list pgsql-performance

From Joel Jacobson
Subject Re: LIMIT causes planner to do Index Scan using a less optimal index
Date
Msg-id o2x8bdec0841004061530g672b7873l9c0bf041569c87ce@mail.gmail.com
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Actually, swapping the order of the conditions did in fact make some difference, strange.

I ran the query a couple of times for each variation to see if the difference in speed was just a coincidence or a pattern. Looks like the speed really is different.

EXPLAIN ANALYZE SELECT TransactionID FROM Transactions WHERE ((AccountID = 108) AND (Currency = 'SEK')) ORDER BY TransactionID LIMIT 1;
                                                                      QUERY PLAN                                                                      
------------------------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.00..116.02 rows=1 width=4) (actual time=1384.401..1384.402 rows=1 loops=1)
   ->  Index Scan using transactions_pkey on transactions  (cost=0.00..1260254.03 rows=10862 width=4) (actual time=1384.399..1384.399 rows=1 loops=1)
         Filter: ((accountid = 108) AND (currency = 'SEK'::bpchar))
 Total runtime: 1384.431 ms
(4 rows)

EXPLAIN ANALYZE SELECT TransactionID FROM Transactions WHERE ((Currency = 'SEK') AND (AccountID = 108)) ORDER BY TransactionID LIMIT 1;
                                                                      QUERY PLAN                                                                      
------------------------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.00..116.02 rows=1 width=4) (actual time=1710.166..1710.167 rows=1 loops=1)
   ->  Index Scan using transactions_pkey on transactions  (cost=0.00..1260254.03 rows=10862 width=4) (actual time=1710.164..1710.164 rows=1 loops=1)
         Filter: ((currency = 'SEK'::bpchar) AND (accountid = 108))
 Total runtime: 1710.200 ms
(4 rows)

EXPLAIN ANALYZE SELECT TransactionID FROM Transactions WHERE ((AccountID = 108) AND (Currency = 'SEK')) ORDER BY TransactionID LIMIT 1;
                                                                      QUERY PLAN                                                                      
------------------------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.00..116.02 rows=1 width=4) (actual time=1366.526..1366.527 rows=1 loops=1)
   ->  Index Scan using transactions_pkey on transactions  (cost=0.00..1260254.03 rows=10862 width=4) (actual time=1366.525..1366.525 rows=1 loops=1)
         Filter: ((accountid = 108) AND (currency = 'SEK'::bpchar))
 Total runtime: 1366.552 ms
(4 rows)

EXPLAIN ANALYZE SELECT TransactionID FROM Transactions WHERE ((Currency = 'SEK') AND (AccountID = 108)) ORDER BY TransactionID LIMIT 1;
                                                                     QUERY PLAN                                                                      
------------------------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.00..116.02 rows=1 width=4) (actual time=1685.395..1685.396 rows=1 loops=1)
   ->  Index Scan using transactions_pkey on transactions  (cost=0.00..1260254.03 rows=10862 width=4) (actual time=1685.394..1685.394 rows=1 loops=1)
         Filter: ((currency = 'SEK'::bpchar) AND (accountid = 108))
 Total runtime: 1685.423 ms
(4 rows)

EXPLAIN ANALYZE SELECT TransactionID FROM Transactions WHERE ((AccountID = 108) AND (Currency = 'SEK')) ORDER BY TransactionID LIMIT 1;
                                                                      QUERY PLAN                                                                      
------------------------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.00..116.02 rows=1 width=4) (actual time=1403.904..1403.905 rows=1 loops=1)
   ->  Index Scan using transactions_pkey on transactions  (cost=0.00..1260254.03 rows=10862 width=4) (actual time=1403.903..1403.903 rows=1 loops=1)
         Filter: ((accountid = 108) AND (currency = 'SEK'::bpchar))
 Total runtime: 1403.931 ms
(4 rows)

EXPLAIN ANALYZE SELECT TransactionID FROM Transactions WHERE ((Currency = 'SEK') AND (AccountID = 108)) ORDER BY TransactionID LIMIT 1;
                                                                      QUERY PLAN                                                                      
------------------------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.00..116.02 rows=1 width=4) (actual time=1689.014..1689.014 rows=1 loops=1)
   ->  Index Scan using transactions_pkey on transactions  (cost=0.00..1260254.03 rows=10862 width=4) (actual time=1689.012..1689.012 rows=1 loops=1)
         Filter: ((currency = 'SEK'::bpchar) AND (accountid = 108))
 Total runtime: 1689.041 ms
(4 rows)

EXPLAIN ANALYZE SELECT TransactionID FROM Transactions WHERE ((AccountID = 108) AND (Currency = 'SEK')) ORDER BY TransactionID LIMIT 1;
                                                                      QUERY PLAN                                                                      
------------------------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.00..116.02 rows=1 width=4) (actual time=1378.322..1378.323 rows=1 loops=1)
   ->  Index Scan using transactions_pkey on transactions  (cost=0.00..1260254.03 rows=10862 width=4) (actual time=1378.320..1378.320 rows=1 loops=1)
         Filter: ((accountid = 108) AND (currency = 'SEK'::bpchar))
 Total runtime: 1378.349 ms
(4 rows)

EXPLAIN ANALYZE SELECT TransactionID FROM Transactions WHERE ((Currency = 'SEK') AND (AccountID = 108)) ORDER BY TransactionID LIMIT 1;
                                                                      QUERY PLAN                                                                      
------------------------------------------------------------------------------------------------------------------------------------------------------
 Limit  (cost=0.00..116.02 rows=1 width=4) (actual time=1696.830..1696.831 rows=1 loops=1)
   ->  Index Scan using transactions_pkey on transactions  (cost=0.00..1260254.03 rows=10862 width=4) (actual time=1696.828..1696.828 rows=1 loops=1)
         Filter: ((currency = 'SEK'::bpchar) AND (accountid = 108))
 Total runtime: 1696.858 ms
(4 rows)



2010/4/6 <Sherry.CTR.Zhu@faa.gov>

I mean the time you spent on prune which one is cheaper might be another cost.
Thanks much!

Xuefeng Zhu (Sherry)
Crown Consulting Inc. -- Oracle DBA
AIM Lab Data Team
(703) 925-3192



Sherry CTR Zhu/AWA/CNTR/FAA
AJR-32, Aeronautical Information Mgmt Group

04/06/2010 03:13 PM

To
Robert Haas <robertmhaas@gmail.com>
cc
Joel Jacobson <joel@gluefinance.com>
Subject
Re: [PERFORM] LIMIT causes planner to do Index Scan using a less         optimal indexLink




Have you tried before?

Thanks much!

Xuefeng Zhu (Sherry)
Crown Consulting Inc. -- Oracle DBA
AIM Lab Data Team
(703) 925-3192



Robert Haas <robertmhaas@gmail.com>

04/06/2010 03:07 PM

To
Sherry CTR Zhu/AWA/CNTR/FAA@FAA
cc
Joel Jacobson <joel@gluefinance.com>
Subject
Re: [PERFORM] LIMIT causes planner to do Index Scan using a less         optimal index





On Tue, Apr 6, 2010 at 3:05 PM, <Sherry.CTR.Zhu@faa.gov> wrote:

Just curious,

Switch the where condition to try to make difference.

how about change
((accountid = 108) AND (currency = 'SEK'::bpchar))
to
( (currency = 'SEK'::bpchar) AND (accountid = 108) ).


In earlier version of Oracle, this was common knowledge that optimizer took the last condition index to use.  

Ignore me if you think this is no sence.  I didn't have a time to read your guys' all emails.  

PostgreSQL doesn't behave that way - it guesses which order will be cheaper.

...Robert




--
Best regards,

Joel Jacobson
Glue Finance

E: jj@gluefinance.com
T: +46 70 360 38 01

Postal address:
Glue Finance AB
Box  549
114 11  Stockholm
Sweden

Visiting address:
Glue Finance AB
Birger Jarlsgatan 14
114 34 Stockholm
Sweden

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