Push down string_agg to pgduck_server

[sfc-gh-okalaci]

Summary

Adds string_agg(text, text) to the list of aggregates shipped to pgduck_server / DuckDB, gated on the delimiter being a non-NULL constant.

This is a customer feature request (Zendesk 11742, previously 10799). Investigation showed the delimiter is the only place string_agg diverges between PostgreSQL and DuckDB:

• NULL delimiter — DuckDB returns NULL, PostgreSQL treats it as "no separator" and concatenates.
• Non-constant (per-row) delimiter — DuckDB errors (Separator argument to StringAgg must be a constant), PostgreSQL evaluates it per row.

With a non-NULL constant delimiter, every other behavior is byte-for-byte identical between the engines: NULL/empty values (dropped/kept the same way), ORDER BY (incl. NULLS FIRST/LAST, multi-key), DISTINCT, FILTER, GROUP BY, unicode, and large inputs. This was verified with an extensive engine-vs-engine torture test (44+ scenarios).

Implementation notes

• IsStringAggShippable receives an Aggref (not a FuncExpr), so it unwraps the delimiter from Aggref->args instead of using GetConstArg(). The delimiter is always the 2nd arg because sort-only junk TargetEntrys are appended after the real args.
• The bytea variant is excluded automatically (DuckDB has no string_agg(BLOB, VARCHAR)), as is any non-constant delimiter.
• Value casts that render differently between engines (e.g. float8::text → 1.0 vs 1) are governed by their own cast-pushdown behavior and are out of scope here.

Test plan

pg_lake_table/tests/pytests/test_string_agg_pushdown.py:

• Pushed + parity vs heap (each with an internal ORDER BY for determinism): basic, ORDER BY DESC, separate sort key with NULLS LAST/FIRST, DISTINCT, FILTER, empty/unicode delimiter, GROUP BY.
• Not pushed + still correct locally: NULL constant delimiter, per-row (column) delimiter, bytea variant.

Locally verified end-to-end via EXPLAIN (VERBOSE) on an Iceberg table: the constant-delimiter case becomes a Custom Scan (Query Pushdown) with string_agg(...) in the remote SQL, while the NULL/column-delimiter cases stay local. Relying on CI for the full pytest run.

Made with Cursor

[sfc-gh-mslot]

we should perhaps open an issue to add a new implementation

[sfc-gh-okalaci]

Opened #441 to track this. The NULL-constant-delimiter case is a straightforward transform (deparse a NULL delimiter as '', since that matches PostgreSQL's "no separator" semantics); the per-row/non-constant delimiter case is harder and captured there too, with examples.