Added function arrayFold()

[Lirikl]

Fixes #34155

Implemention of function arrayFold(x1 ,..., xn, accum -> expression, array1,...,arrayn, init_accum), applying a lambda function to a number of array columns, collecting the result in an accumulator. The accumulator can be either constant or a column.

The implementation gets the columns of the N-th elements for every array and applies them sequentially.

Examples:

SELECT arrayFold(x, acc -> x + acc, [1, 2, 3, 4, 5], toInt64(0)); -- sums the elements of array;
SELECT arrayFold(x + acc.1, x * acc.2), [1, 2, 3, 4, 5], (toInt64(0), toInt64(1)); -- sums and multiplies the elements of array;
SELECT arrayFold(x, acc -> arrayPushFront(ACC, x), [1, 2, 3, 4, 5], emptyArrayInt64()); -- reverses array;

Changelog category :

• New Feature

Changelog entry :

Add function "arrayFold(x1, ..., xn, accum -> expression, array1, ..., arrayn, init_accum)" which applies a lambda function to multiple arrays of the same cardinality and collects the result in an accumulator.

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[clickhouse-ci]

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The error is: More than one changelog category specified: 'New Feature', 'Implementaеion of function arrayFold(x1,...,xn,accum -> expression, array1,...,arrayn, init_accum) applying lambda function to a number of same sized array columns and collecting result in accumulator. Accumulator can be either constant or column.'

[clickhouse-ci]

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The error is: More than one changelog category specified: 'New Feature', '### Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):'

[clickhouse-ci]

This is an automatic comment. The PR descriptions does not match the template.

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The error is: More than one changelog category specified: 'New Feature', '### Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):'

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[rschu1ze]

Thanks a lot! Would you add some docs to docs/en/sql-reference/functions/array-functions.md?

And just out of interest: did you have a look at the previous attempts to add arrayFold? #34155. Asking because there were some performance issues and I am curious how this PR addresses them?

[Lirikl]

Thanks a lot! Would you add some docs to docs/en/sql-reference/functions/array-functions.md?

And just out of interest: did you have a look at the previous attempts to add arrayFold? #34155. Asking because there were some performance issues and I am curious how this PR addresses them?

Yes, I had a look at previous solution, and implemented function in vectorized fashion as was suggested by alexey-milovidov.
I am approximating this solution as at least 100x and some magnitudes faster that implementation in #21589 depending on array sizes based on few tests including those added in performance tests.

In a week I am planning to add docs and some tests comparing this functions against specified functions such as arraySum

[rschu1ze]

Will check the code probably tomorrow. It would be nice if you could add some comments to the code that document the design decisions (so novice readers like me will have an easier time 😄 )

[Lirikl]

Will check the code probably tomorrow. It would be nice if you could add some comments to the code that document the design decisions (so novice readers like me will have an easier time 😄 )

eah, alexey-milovidov had the same comment about the lack of comments, surely will do it too

[rschu1ze]

Don't we need at least three arguments?

[rschu1ze]

Function getLambdaArgumentTypes isn't implemented often in the codebase. Suggest to add some comments what the elements arguments represent. I think they are simply the function arguments (lambda function, array arguments, initial accumulator) but a naive reader could think they are the lambda's arguments.

[rschu1ze]

(toString() is superfluous)

[rschu1ze]

nested_types.back() = arguments.back()?

[rschu1ze]

As written elsewhere, below code is a bit hard to grasp. Let's add comments.