parser: reject DISTINCT ON instead of fabricating an ON() call#54
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`SELECT DISTINCT ON (a) a, b FROM t` was silently mis-parsed: after consuming DISTINCT the select-list parser read `ON (a)` as a function call `ON(a)` occupying the first output slot, then took the intended first column `a` as that call's alias -- dropping a projected column and turning the query into `SELECT DISTINCT ON(a) AS a, b`. DISTINCT ON (a PostgreSQL extension) is not supported. Reject it cleanly right after DISTINCT (error, no AST) rather than producing the wrong tree. Plain DISTINCT (no ON) is unaffected. Tests (test_parser_negative): `DISTINCT ON (...)` is rejected; plain `DISTINCT a, b` still parses. Full suite 37/37. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01FyKDAbMWGwiZq4iJx3imsg
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Summary
Second-pass audit finding F3 (dialect gap, silently-wrong-AST).
SELECT DISTINCT ON (a) a, b FROM twas silently mis-parsed: after consumingDISTINCT, the select-list parser readON (a)as a function callON(a)occupying the first output slot, then took the intended first columnaas that call's alias — dropping a projected column and turning the query intoSELECT DISTINCT ON(a) AS a, b.DISTINCT ON(a PostgreSQL extension) is not supported. Reject it cleanly right afterDISTINCT(error, no AST) rather than producing the wrong tree. PlainDISTINCT(noON) is unaffected.Full
DISTINCT ONsupport (distinct-on expression list + its analyzer/binder legs) is a separate feature, out of scope — this removes the silent mis-parse.Tests (
test_parser_negative)DISTINCT ON (...)is rejected; plainDISTINCT a, bstill parses. Full suite 37/37.