fix: calcite optimization adds LITERAL_AGG(true)

[mbwhite]

fix: calcite optimization adds literal_agg

I was testing running various Calcite optimisations; TPC/H 16 specifically was optimised in such a way that it wasn't possible for the Isthmus SubstraitRelVisitor to handle it.

The issue was that calcite had added LITERAL_AGG(true).

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In Apache Calcite, LITERAL_AGG(true) is an internal aggregate function injected during the decorrelation process (specifically when handling ANY, SOME, or IN subqueries).

Here is why it is there and what it does:

1. The Problem: Handling Empty Subqueries

In your "before" plan, there is a <> SOME(...) correlated subquery. By SQL standards, if a subquery inside a quantified comparison (SOME/ANY) returns zero rows, the entire condition evaluates to FALSE (or UNKNOWN), not NULL.

2. The Solution: Flagging Matches

When Calcite transforms the correlated subquery into a join (LogicalCorrelate / LogicalAggregate), it needs a foolproof way to know if the subquery actually produced any rows for a given correlated key ($cor0.L_PARTKEY).

• LITERAL_AGG(true) evaluates the literal value true for every row entering the aggregate.
• Because it is an aggregate function, if the subquery returns zero rows, LITERAL_AGG(true) will return NULL.
• If the subquery returns one or more rows, it returns true.

3. How Calcite Uses It

Look closely at the massive LogicalFilter(condition=[OR(...)] right above the correlate in your "after" plan.

Calcite uses the output of LITERAL_AGG(true) (which maps to column $19 or $20 in that flattened row) to evaluate the exact short-circuiting logic of the SOME operator:

If it's NULL: The subquery was empty $\rightarrow$ handle as false/null.
If it's TRUE: The subquery returned data $\rightarrow$ proceed to check the actual value comparisons (like COUNT and MAX).

It essentially acts as a highly optimized, null-aware boolean flag for row existence during complex subquery unnesting.

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