Document reduce_agg aggregate function #12195
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wenleix
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| ``combineFunction`` takes two states and returns the new state. | ||
| The final state is returned. | ||
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| ``redcue_agg`` supports state with native container type long, double or boolean. |
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does it mean that S must be one of these?
(just curious, why so?)
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typo: redcue_agg -> reduce_agg
Could you add an example of usage?
| .. function:: reduce_agg(inputValue T, initialState S, inputFunction(S, T, S), combineFunction(S, S, S)) -> S | ||
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| Returns a single value reduced from input values. ``inputFunction`` and ``combineFunction`` will be invoked. | ||
| ``inputFunction`` takes the current state (initially ``initialState``) and the input value, and returns the new state. |
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does it mean reduce_agg runs on single node?
the wording suggests so; but existence of combineFunction suggests it is parallel
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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...
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@findepi, I don't think the name combineFunction implies parallelism.
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@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
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Thanks @mbasmanova and @findepi for reviewing. I added examples, here is how it looks like:
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| ``reduce_agg`` supports state with native container type long, double or boolean. |
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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.
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@dain : Maybe I will just say reduce_agg currently supports boolean, integer (TINYINT, SMALLINT, INTEGER, BIGINT) and floating-point (REAL, DOUBLE) types .
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It also supports dates, times, timestamps, right?
wenleix
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@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. |
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Thanks @electrum for reviewing. Comments addressed. |
Extracted from prestodb/presto#12195
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@electrum David, is this ready to merge? Could you approve? |
| Returns a single value reduced from input values. ``inputFunction`` and ``combineFunction`` will be invoked. | ||
| ``inputFunction`` takes the current state (initially ``initialState``) and the input value, and returns the new state. | ||
| ``combineFunction`` takes two states and returns the new state. | ||
| The value of final state is returned:: |
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typo: there should be only one colon, e.g. "returned: "
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