Обсуждение: Efficiency Problem

Hi all,<br />     I'm composing a query from a web application of type:<br /><br />     <b>SELECT * FROM table WHERE
a_text_fieldLIKE replace_something ('%</b><b><i>a_given_string</i></b><b>%');</b><br /><br /> The function
replace_something(... ) is a stored procedure that replaces some particular characters with others.<br /> The problem
isthat I noticed that this query is inefficient... and I think that the replace_something ( ... ) function is called
foreach row of the table.<br /><br /> This observation is motivated by the fact that it takes around 30 seconds to
executeon the table (of about 25,000 rows), whereas if I execute:<br />     <b>SELECT * FROM table WHERE a_text_field
LIKE'</b><b><i>pre_processed_string</i></b><b><i></i></b><b>';</b><br /><br /> where<i> pre_processed_string</i><i>
</i>isthe result of the application of replace_something ('%<i>a_given_string</i>%')  it just takes 164ms.<br /><br />
Theexecution of <br />     <b>SELECT replace_something ('%</b><b><i>a_given_string</i></b><b>%')</b><br />  takes only
14ms.<br/><br /> Summarizing, <br /> - Replace function:     14ms<br /> - SELECT query without replace function:    
164ms<br/> - SELECT query with replace function:      30.000ms<br /><br /> Morever, I cannot create a stored procedure
thatprecalculate the <i>pre_processed_string </i>and executes the query, since I dinamically <br /> compose other
conditionsin the WHERE clause.<br /><br /> Any suggestion?<br /><br /> Thank you.<br /><i></i> 

Hi,

1) Is function marked as immutable?

2) if immutable doesnt help... It should be possible execute it first, and
use it in other dynamics things in where...

Cheers,

Misa

Sent from my Windows Phone
------------------------------
From: Surfing
Sent: 17/03/2013 12:16
To: pgsql-sql@postgresql.org
Subject: [SQL] Efficiency Problem

  Hi all,
    I'm composing a query from a web application of type:

    *SELECT * FROM table WHERE a_text_field LIKE replace_something ('%**
a_given_string**%');*

The function replace_something( ... ) is a stored procedure that replaces
some particular characters with others.
The problem is that I noticed that this query is inefficient... and I think
that the replace_something ( ... ) function is called for each row of the
table.

This observation is motivated by the fact that it takes around 30 seconds
to execute on the table (of about 25,000 rows), whereas if I execute:
    *SELECT * FROM table WHERE a_text_field LIKE '**pre_processed_string****
';*

where* pre_processed_string** *is the result of the application of
replace_something ('%*a_given_string*%')  it just takes 164ms.

The execution of
    *SELECT replace_something ('%**a_given_string**%')*
 takes only 14ms.

Summarizing,
- Replace function:     14ms
- SELECT query without replace function:     164ms
- SELECT query with replace function:      30.000ms

Morever, I cannot create a stored procedure that precalculate the
*pre_processed_string
*and executes the query, since I dinamically
compose other conditions in the WHERE clause.

Any suggestion?

Thank you.
**

IMMUTABLE solved the problem.

Thank you!

Hi,

1) Is function marked as immutable?

2) if immutable doesnt help... It should be possible execute it first, and use it in other dynamics things in where...

Cheers,

Misa

Sent from my Windows Phone

---

From: Surfing
Sent: 17/03/2013 12:16
To: pgsql-sql@postgresql.org
Subject: [SQL] Efficiency Problem

Hi all,
    I'm composing a query from a web application of type:

    SELECT * FROM table WHERE a_text_field LIKE replace_something ('%a_given_string%');

The function replace_something( ... ) is a stored procedure that replaces some particular characters with others.
The problem is that I noticed that this query is inefficient... and I think that the replace_something ( ... ) function is called for each row of the table.

This observation is motivated by the fact that it takes around 30 seconds to execute on the table (of about 25,000 rows), whereas if I execute:
    SELECT * FROM table WHERE a_text_field LIKE 'pre_processed_string';

where pre_processed_string is the result of the application of replace_something ('%a_given_string%')  it just takes 164ms.

The execution of
    SELECT replace_something ('%a_given_string%')
 takes only 14ms.

Summarizing,
- Replace function:     14ms
- SELECT query without replace function:     164ms
- SELECT query with replace function:      30.000ms

Morever, I cannot create a stored procedure that precalculate the pre_processed_string and executes the query, since I dinamically
compose other conditions in the WHERE clause.

Any suggestion?

Thank you.

---

From: Surfing
Sent: 17/03/2013 12:16
To: pgsql-sql@postgresql.org
Subject: [SQL] Efficiency Problem

Hi all,
    I'm composing a query from a web application of type:

    SELECT * FROM table WHERE a_text_field LIKE replace_something ('%a_given_string%');

The function replace_something( ... ) is a stored procedure that replaces some particular characters with others.
The problem is that I noticed that this query is inefficient... and I think that the replace_something ( ... ) function is called for each row of the table.

This observation is motivated by the fact that it takes around 30 seconds to execute on the table (of about 25,000 rows), whereas if I execute:
    SELECT * FROM table WHERE a_text_field LIKE 'pre_processed_string';

where pre_processed_string is the result of the application of replace_something ('%a_given_string%')  it just takes 164ms.

The execution of
    SELECT replace_something ('%a_given_string%')
 takes only 14ms.

Summarizing,
- Replace function:     14ms
- SELECT query without replace function:     164ms
- SELECT query with replace function:      30.000ms

Morever, I cannot create a stored procedure that precalculate the pre_processed_string and executes the query, since I dinamically
compose other conditions in the WHERE clause.

Any suggestion?

Thank you.