fix(sql): remove constants in order_by calls during select merging

[cpcloud]

Remove literals in ORDER BY when sorting tables. Fixes #10428.

[cpcloud]

It looks like DuckDB's planner erases constants in ORDER BY:

❯ duckdb <<< "SET order_by_non_integer_literal=true; CREATE TABLE t AS SELECT i FROM RANGE(1, 4) AS _ (i); EXPLAIN SELECT i, 'foo' as x FROM t ORDER BY i DESC, 'foo'"

┌─────────────────────────────┐
│┌───────────────────────────┐│
││       Physical Plan       ││
│└───────────────────────────┘│
└─────────────────────────────┘
┌───────────────────────────┐
│         PROJECTION        │
│    ────────────────────   │
│__internal_decompress_integ│
│     ral_bigint(#0, 1)     │
│__internal_decompress_strin│
│           g(#1)           │
│                           │
│          ~3 Rows          │
└─────────────┬─────────────┘
┌─────────────┴─────────────┐
│          ORDER_BY         │
│    ────────────────────   │
│          t.i DESC         │
└─────────────┬─────────────┘
┌─────────────┴─────────────┐
│         PROJECTION        │
│    ────────────────────   │
│__internal_compress_integra│
│     l_utinyint(#0, 1)     │
│__internal_compress_string_│
│        uinteger(#1)       │
│                           │
│          ~3 Rows          │
└─────────────┬─────────────┘
┌─────────────┴─────────────┐
│         PROJECTION        │
│    ────────────────────   │
│             i             │
│             x             │
│                           │
│          ~3 Rows          │
└─────────────┬─────────────┘
┌─────────────┴─────────────┐
│         SEQ_SCAN          │
│    ────────────────────   │
│             t             │
│                           │
│       Projections: i      │
│                           │
│          ~3 Rows          │
└───────────────────────────┘

[kszucs]

Can we remove all expressions which are constant across the relation? I am thinking of all pure scalar nodes.

[cpcloud]

What do you mean? We can't just start deleting anything that's a literal scalar. Expressions like

SELECT 1 as a
FROM t

are 100% valid and useful.

[kszucs]

I only mean the order by expressions.

[cpcloud]

No because it's possible to write things like 1 if x else 0, and use them in sort keys.

[kszucs]

expr = 1 if x else 0 right, though we can identify that as well, like its .relations attribute is not empty

[kszucs]

In any case, it doesn't block this PR.