Re: using custom scan nodes to prototype parallel sequential scan

On Fri, Nov 14, 2014 at 02:51:32PM +1300, David Rowley wrote:
> Likely for most aggregates, like count, sum, max, min, bit_and and bit_or the
> merge function would be the same as the transition function, as the state type
> is just the same as the input type. It would only be aggregates like avg(),
> stddev*(), bool_and() and bool_or() that would need a new merge function
> made... These would be no more complex than the transition functions... Which
> are just a few lines of code anyway.
> 
> We'd simply just not run parallel query if any aggregates used in the query
> didn't have a merge function.
> 
> When I mentioned this, I didn't mean to appear to be placing a road block.I was
> just bringing to the table the information that COUNT(*) + COUNT(*) works ok
> for merging COUNT(*)'s "sub totals", but AVG(n) + AVG(n) does not.

Sorry, late reply, but, FYI, I don't think our percentile functions
can't be parallelized in the same way:
test=> \daS *percent*                                                      List of aggregate functions   Schema   |
Name       |  Result data type  |             Argument data types              |
Description------------+-----------------+--------------------+----------------------------------------------+-------------------------------------
pg_catalog| percent_rank    | double precision   | VARIADIC "any" ORDER BY VARIADIC "any"       | fractional rank of
hypotheticalrow pg_catalog | percentile_cont | double precision   | double precision ORDER BY double precision   |
continuousdistribution percentile pg_catalog | percentile_cont | double precision[] | double precision[] ORDER BY
doubleprecision | multiple continuous percentiles pg_catalog | percentile_cont | interval           | double precision
ORDERBY interval           | continuous distribution percentile pg_catalog | percentile_cont | interval[]         |
doubleprecision[] ORDER BY interval         | multiple continuous percentiles pg_catalog | percentile_disc | anyelement
       | double precision ORDER BY anyelement         | discrete percentile pg_catalog | percentile_disc | anyarray
     | double precision[] ORDER BY anyelement       | multiple discrete percentiles
 

--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com
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