Re: [PERFORM] which one is faster

2010-10-26 Thread Szymon Guz 
On 26 October 2010 12:56, AI Rumman rumman...@gmail.com wrote:

 Which one is faster?
 select count(*) from talble
 or
 select count(id) from table
 where id is the primary key.



Check the query plan, both queries are the same.

regards
Szymon

Re: [PERFORM] which one is faster

2010-10-26 Thread Marcin Mirosław 
W dniu 26.10.2010 12:59, Szymon Guz pisze:
 both queries are the same.

IMHO they aren't the same, but they returns the same value in this case.
I mean count(field) doesn't count NULL values, count(*) does it.
I'm writing this only for note:)
Regards

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Re: [PERFORM] which one is faster

2010-10-26 Thread Szymon Guz 
2010/10/26 Marcin Mirosław mar...@mejor.pl

 W dniu 26.10.2010 12:59, Szymon Guz pisze:
  both queries are the same.

 IMHO they aren't the same, but they returns the same value in this case.
 I mean count(field) doesn't count NULL values, count(*) does it.
 I'm writing this only for note:)
 Regards


Yup, indeed. I omitted that note, as it was written that the field is
primary key :).

regards
Szymon

Re: [PERFORM] which one is faster

2010-10-26 Thread Grzegorz Jaśkiewicz 
implementation wise, count(*) is faster. Very easy to test:

SELECT COUNT(*) FROM generate_series(1,100) a, generate_series(1,1000) b;

SELECT COUNT(a) FROM generate_series(1,100) a, generate_series(1,1000) b;


;]

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Re: [PERFORM] which one is faster

2010-10-26 Thread Szymon Guz 
2010/10/26 Grzegorz Jaśkiewicz gryz...@gmail.com

 implementation wise, count(*) is faster. Very easy to test:

 SELECT COUNT(*) FROM generate_series(1,100) a, generate_series(1,1000) b;

 SELECT COUNT(a) FROM generate_series(1,100) a, generate_series(1,1000) b;


 ;]


Well, strange. Why is that slower?

Re: [PERFORM] which one is faster

2010-10-26 Thread Szymon Guz 
2010/10/26 Grzegorz Jaśkiewicz gryz...@gmail.com

 2010/10/26 Szymon Guz mabew...@gmail.com:
 
  Well, strange. Why is that slower?

 To answer that fully, you would need to see the implementation.
 suffice to say,

 count(a) does:

 if (a  NULL)
 {
  count++;
 }

 and count(*) does:

  count++;



Yup, I was afraid of that, even if there is not null on the column... but I
think usually nobody notices the difference with count.

regards
Szymon

Re: [PERFORM] which one is faster

2010-10-26 Thread Mladen Gogala 

On 10/26/2010 6:56 AM, AI Rumman wrote:

Which one is faster?
select count(*) from talble
or
select count(id) from table
where id is the primary key.
PostgreSQL doesn't utilize the access methods known as FULL INDEX SCAN 
and FAST FULL INDEX SCAN, so the optimizer will generate the 
sequential scan in both cases. In other words, PostgreSQL will read the 
entire table when counting, no matter what.


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Sr. Oracle DBA
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[PERFORM] Which one is faster: one way reading =single pass reading

2005-03-20 Thread Rosny 
Hi,

Which one is faster: one way reading =single pass reading
Assumption :
a. Need to have 3 millions records
b. Need to have call 10  or 20 records repeatly
  (so for database it will be 10 times connection, each connection with one
record.
   or can be fancy 1 connection call return 10 sets of records)


1. Reading from Flat file
   Assume already give file name and just need to read the file
  (since it is flat file, each record represent a filename, with multiple
directory category)

2. Reading from XML file
   Assume schema already given just need to read the file
  (since it is xml file, each record represent an xml filename, with
multiple directory category)

3. Reading from Postgresql
   Assume primary key has been done with indexing
   just need to search the number and grap the text content
  (assume 3 millions of records, search the number, read the content file)

trying to recreate WebDBReader (from nutch) using C#
http://nutch.sourceforge.net/docs/api/net/nutch/db/WebDBReader.html

Thank you in advances,
Rosny





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