Re: LIMIT causes planner to do Index Scan using a less optimal index

Guys,

Thanks for trying and opening your mind.
If you want to know how Oracle addressed this issue, here it is: index
on two columns. I remember that they told me in the training postgres has
no this kind of index, can someone clarify?

Thanks much!

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

Joel Jacobson <joel(at)gluefinance(dot)com>
04/06/2010 06:30 PM

To
Sherry CTR Zhu/AWA/CNTR/FAA(at)FAA, pgsql-performance(at)postgresql(dot)org
cc
Robert Haas <robertmhaas(at)gmail(dot)com>
Subject
Re: [PERFORM] LIMIT causes planner to do Index Scan using a less optimal
index

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(dot)CTR(dot)Zhu(at)faa(dot)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(at)gmail(dot)com>
cc
Joel Jacobson <joel(at)gluefinance(dot)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(at)gmail(dot)com>
04/06/2010 03:07 PM

To
Sherry CTR Zhu/AWA/CNTR/FAA(at)FAA
cc
Joel Jacobson <joel(at)gluefinance(dot)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(dot)CTR(dot)Zhu(at)faa(dot)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(at)gluefinance(dot)com
T: +46 70 360 38 01

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Glue Finance AB
Box 549
114 11 Stockholm
Sweden

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