[SPARK-52798] [SQL] Add function approx_top_k_combine

[yhuang-db]

What changes were proposed in this pull request?

This PR adds a SQL function: approx_top_k_accumulate, an aggregation function that merges multiple sketches into a single sketch.

Syntax

approx_top_k_combine(expr[, maxItemsTracked])

Arguments

• expr: An expression of sketch structs
• maxItemsTracked: An optional INTEGER literal. If maxItemsTracked is specified, use this value for the newly generated combined sketch. If maxItemsTracked is not specified, all input sketches must have the same maxItemsTracked, and the output sketch would use the same value as well.

Returns

The return of this function is a STRUCT with four fields: sketch, itemDataType, maxItemsTracked and typeCode. The return is exactly the same as for approx_top_k_accumulate.

Why are the changes needed?

They are useful sibling functions for approx_top_k queries.

Does this PR introduce any user-facing change?

Yes, this PR introduces a new user-facing SQL function. See user examples as below.

SELECT approx_top_k_estimate(approx_top_k_combine(sketch, 10000), 5) FROM (SELECT approx_top_k_accumulate(expr) AS sketch FROM VALUES (0), (0), (1), (1) AS tab(expr) UNION ALL SELECT approx_top_k_accumulate(expr) AS sketch FROM VALUES (2), (3), (4), (4) AS tab(expr))

How was this patch tested?

Unit tests for end-to-end SQL queries and invalid input for expressions.

Was this patch authored or co-authored using generative AI tooling?