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Implementation of GROUPING SETS (T431: Extended grouping capabilities)

From: Олег Царев <zabivator(at)gmail(dot)com>
To: pgsql-hackers(at)postgresql(dot)org
Subject: Implementation of GROUPING SETS (T431: Extended grouping capabilities)
Date: 2009-05-10 12:57:06
Message-ID: (view raw or whole thread)
Lists: pgsql-hackers
Hello all.
Please, approve my ideas for implementation.

Standart has feature T431: Extended grouping capabilities.
This feature i found in TODO-list: -> SQL Commands -> TO DO

MS SQL 2005 partial support this feature:

MS SQL 2008 support this feature:

Oracle support this feature:

So, it's short notes about GROUPING SETS, but more complete
information have in a official documentation of MS SQL and Oracle
(copyright limited for send as attach).


select A,B,C,SUM(D) from table group by GROUPING SETS( (A,B,C), (A),
() ) - it's example of use grouping sets.
Semantic of this construction - make group by over source more, than
one group of column.
It's very wide key - A,B C. In result set of this example we can find
result set of select   select A,B,C,SUM(D) from table group by A,B,C -
as subset. It's mind: "GROUP BY A,B,C" - subset of "GROUP BY GROUPING
SETS( (A,B,C), (A), () )
Two subset - is GROUP BY A B, and instead C column we look NULL.
Third subset - GROUP BY (), instead A,B,C - NULL, one row - it's name
"GRAND TOTAL". - calculate over all subset without grouping

Also have function "GROUPING"  it's function say about null - "real
null" (from table) or generated by "GROUP BY GROUPING SETS"

My point: this feature can implement over GROUP BY and UNION ALL
We can make decomposition of "GROUP BY GROUPING SETS( (A,B,C),(A),()
)" to  select A,B,C fron table GROUP BY A,B,C .UNION  ALL select
A,B,NULL from table group BY A,B UNION ALL NUll,NUll,NULL from table
group by();

So, it's very simple, don't require modification of executor and
callibrate cost - only parser and semantic anylysis,
So, ROLLUP(A1,...An) is alias to "GROUP BY GROUPING SETS(
(A1,...,An),(A1,...,An-1),... (An-1,An),(An),() ),
CUBE - analogue.

If this idea it's good -  i can write code base on old patch or
from clean list (as you wish).

In future i know how to implement ROLLUP more optimal (executor
iterator) and use this ROLLUP for optimisation another GROUP BY,



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Subject: Re: Implementation of GROUPING SETS (T431: Extended grouping capabilities)
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