Document reduce_agg aggregate function

[wenleix]

No description provided.

[findepi]

does it mean that S must be one of these?

(just curious, why so?)

[mbasmanova]

typo: redcue_agg -> reduce_agg

Could you add an example of usage?

[wenleix]

@findepi : Correct, see the comments in code:

presto/presto-main/src/main/java/com/facebook/presto/operator/aggregation/ReduceAggregationFunction.java

Lines 124 to 127 in 79f480b

	// State with Slice or Block as native container type is intentionally not supported yet,
	// as it may result in excessive JVM memory usage of remembered set.
	// See JDK-8017163.
	throw new UnsupportedOperationException();

Also see #9553

[findepi]

does it mean reduce_agg runs on single node?
the wording suggests so; but existence of combineFunction suggests it is parallel

[wenleix]

No it runs in parallel. Any thoughts about how to revise the wording? -- Since when I read it, I didn't see it will run on single node, but I am obviously biased...

[dain]

@findepi, I don't think the name combineFunction implies parallelism.

[findepi]

@dain not the name -- the existence of it.
It it was single-threaded, there would be just initialSate and inputFunction (aka "accumulator" or "accumulatingFunction").
The need for combineFunction arises only when you want to merge two branches of processing (ie parallel computations).

[wenleix]

Thanks @mbasmanova and @findepi for reviewing. I added examples, here is how it looks like:

[dain]

I don't think any end user will understand what this means. Maybe state this as reduce_agg currently does not support variable width types, container types, or inexact types.

[wenleix]

@dain : Maybe I will just say reduce_agg currently supports boolean, integer (TINYINT, SMALLINT, INTEGER, BIGINT) and floating-point (REAL, DOUBLE) types .

[findepi]

It also supports dates, times, timestamps, right?

[nezihyigitbasi]

@findepi @dain @mbasmanova Is there anything else that's needed from this PR? We need to merge this and update the release notes PR (#12194) with a link to this new doc.

[wenleix]

Thanks @electrum for reviewing. Comments addressed.

[mbasmanova]

@electrum David, is this ready to merge? Could you approve?

[mbasmanova]

typo: there should be only one colon, e.g. "returned: "

[wenleix]

@mbasmanova : This is Sphinx syntax, e.g. https://github.com/prestodb/presto/blame/5c6bf5f8893a3ac7080b199667f23b480ff54983/presto-docs/src/main/sphinx/functions/array.rst#L122