On 12/29/15 6:50 PM, Tom Lane wrote:
Jim Nasby<Jim.Nasby@xxxxxxxxxxxxxx> writes:
>BTW, if you want to simply apply a function to all elements in an array
>there is an internal C function array_map that can do it. There's no SQL
>interface to it, but it shouldn't be hard to add one.
That wouldn't be useful for the example given originally, since it
iterates over just one array not two arrays in parallel. But you could
imagine writing something similar that would iterate over two arrays and
call a two-argument function.
Actually, I suspect you could pretty easily do array_map(regprocedure,
VARIADIC anyarray).
Whether it's worth a SQL interface is debatable though. Whatever
efficiency you might gain from using this would probably be eaten by the
overhead of calling a SQL or PL function for each pair of array elements.
You'd probably end up in the same ballpark performance-wise as the UNNEST
solution given earlier.
Take a look at [1]; using a rough equivalent to array_map is 6% faster
than unnest().
The array op array version is 30% faster that plpgsql, which based on
the code at [2] I assume is doing
explain analyze select array(select a*b from unnest(array(select
random() from generate_series(1,1000000)), array(select random() from
generate_series(1,1000000)))) u(a,b);
The syntactic sugar of r := array_map('function(a, b)', in1, in2) (let
alone r := in1 * in2;) is appealing too.
[1]
http://theplateisbad.blogspot.com/2015/12/the-arraymath-extension-vs-plpgsql.html
[2]
http://theplateisbad.blogspot.com/2015/12/more-fortran-90-like-vector-operations.html
--
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
Data in Trouble? Get it in Treble! http://BlueTreble.com
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