Support an in-call LIMIT clause for ARRAY_AGG() - #4600
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`ARRAY_AGG(«expr» LIMIT «n»)` restricts the number of elements collected into the array per group. Rows past the limit are neither converted nor collected by the accumulator, although they still have to be consumed in order to determine the group boundary. The `LIMIT` clause must come last, after the `{RESPECT|IGNORE} NULLS` and in-call `ORDER BY` clauses.
`LIMIT` can be used as a guard to impose a bound on both the arrays held in memory and the partial state serialized into a continuation. In conjunction with an in-call `ORDER BY` clause (which is anticipated by the grammar but not yet supported), `LIMIT` is also a convenient way to produce “top N” lists.
Regarding the grammar, note that the new `aggregateLimitClause` (which is admitted for `ARRAY_AGG()` only) is deliberately narrower than the existing (but unsupported) `limitClause` of a `SELECT` query: It takes no `OFFSET`, and `«n»` must be a literal, since the limit gets baked into the plan rather than bound per execution.
Resolves #4599.
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ARRAY_AGG(«expr» LIMIT «n»)restricts the number of elements collected into the array per group. Rows past the limit are neither converted nor collected by the accumulator, although they still have to be consumed in order to determine the group boundary. TheLIMITclause must come last, after the{RESPECT|IGNORE} NULLSand in-callORDER BYclauses.LIMITcan be used as a guard to impose a bound on both the arrays held in memory and the partial state serialized into a continuation. In conjunction with an in-callORDER BYclause (which is anticipated by the grammar but not yet supported),LIMITis also a convenient way to produce “top N” lists.Regarding the grammar, note that the new
aggregateLimitClause(which is admitted forARRAY_AGG()only) is deliberately narrower than the existing (but unsupported)limitClauseof aSELECTquery: It takes noOFFSET, and«n»must be a literal, since the limit gets baked into the plan rather than bound per execution.Resolves #4599.