[postgis-tickets] [PostGIS] #3751: Parallel support for aggregates with transitions

[PostGIS]

#3751: Parallel support for aggregates with transitions
-------------------------+---------------------------
 Reporter:  robe         |      Owner:  pramsey
     Type:  enhancement  |     Status:  new
 Priority:  high         |  Milestone:  PostGIS 2.4.0
Component:  postgis      |    Version:  2.3.x
 Keywords:               |
-------------------------+---------------------------
 I didn't think aggregate functions that use transitions (take internal as
 input) are parallelizable.

 Turns out that even in PostgreSQL 9.6 it is possible to do so as long as
 you define a serialize and deserialize function for them.  These are new
 args to CREATE AGGREGATE in 9.6

 https://www.postgresql.org/docs/9.6/static/sql-createaggregate.html


 {{{
 CREATE AGGREGATE name ( [ argmode ] [ argname ] arg_data_type [ , ... ] )
 (
     SFUNC = sfunc,
     STYPE = state_data_type
     [ , SSPACE = state_data_size ]
     [ , FINALFUNC = ffunc ]
     [ , FINALFUNC_EXTRA ]
     [ , COMBINEFUNC = combinefunc ]
     [ , SERIALFUNC = serialfunc ]
     [ , DESERIALFUNC = deserialfunc ]
     [ , INITCOND = initial_condition ]
     [ , MSFUNC = msfunc ]
     [ , MINVFUNC = minvfunc ]
     [ , MSTYPE = mstate_data_type ]
     [ , MSSPACE = mstate_data_size ]
     [ , MFINALFUNC = mffunc ]
     [ , MFINALFUNC_EXTRA ]
     [ , MINITCOND = minitial_condition ]
     [ , SORTOP = sort_operator ]
     [ , PARALLEL = { SAFE | RESTRICTED | UNSAFE } ]
 )
 }}}

 From the docs


 ----
 serialfunc

     An aggregate function whose state_data_type is internal can
 participate in parallel aggregation only if it has a serialfunc function,
 which must serialize the aggregate state into a bytea value for
 transmission to another process. This function must take a single argument
 of type internal and return type bytea. A corresponding deserialfunc is
 also required.
 deserialfunc

     Deserialize a previously serialized aggregate state back into
 state_data_type. This function must take two arguments of types bytea and
 internal, and produce a result of type internal. (Note: the second,
 internal argument is unused, but is required for type safety reasons.)

 ----


 It would be really nice if we can do this because then we have the
 possiblity of making really costly aggregates like ST_Union  and the
 various raster aggregates parallelizable.

--
Ticket URL: <https://trac.osgeo.org/postgis/ticket/3751>
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