Обсуждение: query decorrelation in postgres

Hello,<br /><br />I am a master's student in computer science at IIT Bombay. As part of my project, I need to get a
decorrelatedversion of a SQL query.  Please could anyone let me know if we have query decorrelation feature implemented
inpostgres ?<br /><br />Thanks,<br />Mahendra Chavan<br /><br /> 

mahendra chavan <mahcha@gmail.com> wrote:

> I am a master's student in computer science at IIT Bombay. As part of my
> project, I need to get a decorrelated version of a SQL query.  Please could
> anyone let me know if we have query decorrelation feature implemented in
> postgres ?

What do you mean by "query decorrelation"? Is it an addtional method for
query optimization? At least there is no word 'decorrelation' in
the postgres documentation.

Regards,
---
ITAGAKI Takahiro
NTT Open Source Software Center

mahendra chavan <mahcha@gmail.com> wrote: 
> What I meant by de-correlation was optimizing a query to get rid of
> sub-queirs by using joins.
> 
> eg. In the TPC-H schema, a query to find out the names of suppliers
> who supply parts having size  < 100
> 
> *Query with nested subqueries:*
> 
> SELECT
>     S_NAME
> FROM
>     SUPPLIER
> WHERE
>     S_SUPPKEY
>     IN (
>         SELECT
>             PS_SUPPKEY
>         FROM
>             PARTSUPP
>         WHERE
>             PS_PARTKEY
>                 IN (
>                     SELECT
>                         P_PARTKEY
>                     FROM
>                         PART
>                     WHERE
>                         P_SIZE < 100
>                 )
> 
> 
> 
> *Query with joins without subqueries:*
> 
> 
> SELECT
>     S_NAME
> FROM
>     SUPPLIER
> INNER JOIN
>     PARTSUPP
> ON
>     S_SUPPKEY = PS_SUPPKEY
> INNER JOIN
>     PART
> ON
>     P_PARTKEY = PS_PARTKEY
> WHERE
>     P_SIZE < 100
> 
Those two queries aren't exactly identical, because you could get
duplicate rows in the second which would not be there in the first. 
Optimizations to "pull up" subqueries into a higher level FROM clause
as joins have been in PostgreSQL for as long as I've been using it,
but the ability to do the specific optimization you show there
(without the duplicates) was added in version 8.4 using "semi-joins". 
I don't think any syntax was added to explicitly write a query using
semi-joins, though; it's an optimization of EXISTS, IN, ANY, etc.
To see how the planner chooses to execute a given query against a
particular schema which has a particular set of statistics about the
data distributions, use the EXPLAIN option.
http://www.postgresql.org/docs/8.4/interactive/sql-explain.html
-Kevin