Add function arrayReduceInRanges

[hczhcz]

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Changelog category (leave one):

• New Feature

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Add function arrayReduceInRanges which aggregates array elements in given ranges.

Detailed description / Documentation draft:

Applies an aggregate function to array elements in given ranges and returns an array containing the result corresponding to each range. The function will return the same result as multiple arrayReduce(agg_func, arraySlice(arr1, index, length), ...).

Example:

SELECT arrayReduceInRanges(
   'sum',
   [1, 2],
   [2, 3],
   [100, 20, 3000, 4]
)

┌─arrayReduceInRanges('sum', [1, 2], [2, 3], [100, 20, 3000, 4])─┐
│ [120,3024]                                                     │
└────────────────────────────────────────────────────────────────┘

[amosbird]

The example is a bit misleading, as I interpret it as supplying two ranges : [1, 2] and [2, 3] at first sight.

[hczhcz]

This one might be better.

SELECT arrayReduceInRanges(
    'sum',
    [1, 2, 3, 4],
    [5, 3, 4, 4],
    [1000000, 200000, 30000, 4000, 500, 60, 7]
) AS res

┌─res─────────────────────────┐
│ [1234500,234000,34560,4567] │
└─────────────────────────────┘

[qoega]

Will it be more clear if you use array of tuples?

SELECT arrayReduceInRanges(
    'sum',
    [(1, 5), (2, 3), (3, 4), (4, 4)],
    [1000000, 200000, 30000, 4000, 500, 60, 7]
) AS res

If you have a case, when index and length are in separate columns you can just use arrayZip

SELECT
    [1, 2, 3, 4, 5] AS x,
    [1, 2, 3, 4, 5] AS y,
    arrayZip(x, y) AS z

┌─x───────────┬─y───────────┬─z───────────────────────────────┐
│ [1,2,3,4,5] │ [1,2,3,4,5] │ [(1,1),(2,2),(3,3),(4,4),(5,5)] │
└─────────────┴─────────────┴─────────────────────────────────┘

[qoega]

It is better to explicitly describe parameters.
By the way there is a function doc template:
https://github.com/ClickHouse/ClickHouse/blob/master/docs/dscr-templates/template-function.md

[alexey-milovidov]

Sorry, it looks like this PR was not merged with upstream properly. Could you please rebase your commits?

[alexey-milovidov]

This is wrong. If constructor has thrown an exception, destructor must not be called.

[alexey-milovidov]

I will merge to read the algorithm more carefully (it's difficult to understand).