Re: [Postgres-xc-developers] CREATE AGGREGATE

Andrei;

Thank you very much for detailed description of the spec and example test.

---
Koichi

(2010年06月23日 15:31), Andrei Martsinchyk wrote:
> Hi Sudo-san,
>
> Good description of CREATE AGGREGATE can be found in the Postgres docs:
>
> http://www.postgresql.org/docs/8.4/static/sql-createaggregate.html
>
> Postgres-XC added additional step (collection function) between
> transition and final calculation functions, and schema
>
> sfunc( internal-state, next-data-values ) --->  next-internal-state
> ffunc( internal-state ) --->  aggregate-value
>
> was changed to
>
> sfunc( node-internal-state, next-data-values ) --->  node-internal-state
> cfunc( coord-internal-state, node-internal-state ) --->  coord-internal-state
> ffunc( coord-internal-state ) --->  aggregate-value
>
> To reflect this change following parameters was added to CREATE AGGREGATE:
>
> CFUNC - collection function,
> CTYPE - collection data type,
> INITCOLLECT - (optional) initial collection value.
>
> It is hard to develop aggregate function from the scratch. For my
> testing I recreated existing functions under different name.
> Example:
>
> create aggregate mymin (int4) (sfunc=int4smaller, stype=int4,
> cfunc=int4smaller, ctype=int4);
>
> You can see the list of defined aggregate functions querying pg_aggregate table:
>
> select * pg_aggregate;
>
> You can also try and construct own aggregate function. You would need
> to choose functions for sfunc, cfunc and ffunc (component functions)
> from already defined in Postgres.
> You can see the list of defined functions querying pg_proc table:
>
> select * pg_proc;
>
> There are requirements for return types and parameter types of the
> component functions.
> cfunc:
> data type of the first parameter must be the same as return type. Data
> types of remaining parameters must be the same as data types of the
> aggregate  function
> sfunc:
> must have two parameters, data type of first parameter must be the
> same as return type of sfunc, data type of second parameter must be
> the same as return type of cfunc
> ffunc:
> must have one parameter, data type must be the same as return type of sfunc.
>
> And finally data types, you can see the list of defined data types
> querying pg_type table:
> select * pg_type;
>
> Feel free to ask if you have a question.
>