fix(optimizer)!: normalization of synthesized output aliases by fivetran-kwoodbeck · Pull Request #7816 · tobymao/sqlglot

fivetran-kwoodbeck · 2026-06-29T18:31:00Z

When the optimizer generates an output alias for an unaliased projection (in qualify_outputs), the synthesized alias does not conform to the dialect's NORMALIZATION_STRATEGY

The patch calls normalize_identifier to ensure the alias follow the dialect's casing rules (e.g. upper-folding in Snowflake). An update to star expansion (_expand_stars) was needed so that columns expanded from quoted source projections carry the quoted flag, keeping star output consistent with an explicit reference to the same column.

Quoted columns keep their exact spelling, since folding them would change which column they resolve to.

Example: Snowflake (upper-folding)

SELECT OBJECT_CONSTRUCT(*) FROM (SELECT a, b FROM x) AS t;

Before

SELECT OBJECT_CONSTRUCT(*) AS _col_0 FROM (SELECT a AS a, b AS b FROM x AS x) AS t;

After

SELECT OBJECT_CONSTRUCT(*) AS _COL_0 FROM (SELECT a AS A, b AS B FROM x AS x) AS t;

georgesittas · 2026-06-30T12:55:00Z

@fivetran-kwoodbeck can you remind me what the motivation behind this was? Was this a bug, like changing the semantics of the output compared to the input? Or is it just an aesthetic/consistency improvement?

fivetran-kwoodbeck · 2026-06-30T14:15:51Z

@fivetran-kwoodbeck can you remind me what the motivation behind this was? Was this a bug, like changing the semantics of the output compared to the input? Or is it just an aesthetic/consistency improvement?

@georgesittas The motivation is to ensure the optimizer both idempotent and to make sure the generated aliases are consistent with each dialect's NORMALIZATION_STRATEGY. If idempotency is a requirement for the optimizer, then yes, it's a bug.

georgesittas · 2026-07-01T12:10:34Z

+            quoted_columns = (
+                {s.output_name: _output_identifier_quoted(s) for s in source_expression.selects}
+                if isinstance(source_expression, exp.Query)
+                else {}
+            )


Nit: I'd make this a set.

Suggested change

quoted_columns = (

{s.output_name: _output_identifier_quoted(s) for s in source_expression.selects}

if isinstance(source_expression, exp.Query)

else {}

)

quoted_columns = {

s.output_name

for s in source_expression.selects

if isinstance(source_expression, exp.Query) and _output_identifier_quoted(s)

}

By the way, is this costly to construct within the for table in tables loop instead of once? Doesn't this repeat work? Is there a better way to implement this?

I'm not sure there's a better way, for example, I don't see if (or how) to reliably access this in the for name in columns: loop.

Yeah feel free to ignore my last comment above. It's fine.

That snippet crashes the test suite. The if isinstance(source_expression, exp.Query) needs to run outside of the loop, not inside.

georgesittas · 2026-07-01T12:10:56Z

-def qualify_outputs(scope_or_expression: Scope | exp.Expr) -> None:
+def qualify_outputs(scope_or_expression: Scope | exp.Expr, dialect: Dialect | None = None) -> None:


@fivetran-kwoodbeck let's pass TSQL() to the qualify_outputs call in qualify_derived_table_outputs. I don't think the dialect should be optional here; the normalize_identifier call in L998 should be unconditional.

georgesittas · 2026-07-01T12:34:34Z

+             "source"."name" AS "NAME",
+             "source"."payload" AS "PAYLOAD"


This is incorrect @fivetran-kwoodbeck. The schema contains the case-sensitive "name" and "payload". The test was explicitly asserting that the projected columns, after expanding stars, retains the case-sensitivity.

Observe:

(sqlglot) ➜ sqlglot git:(optimizer/bug-normalizealiases) ✗ runsf "create table test as select 1 as \"c\"; select * from test; select \"C\" from test" create table test as select 1 as "c"; +----------------------------------+ | status | |----------------------------------| | Table TEST successfully created. | +----------------------------------+ select * from test; +---+ | c | |---| | 1 | +---+ select "C" from test ╭─ Error ─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╮ │ 000904 (42000): 01c56ad2-0009-d3dd-0001-b9fe095fc206: SQL compilation error: error line 1 at position 7 │ │ invalid identifier 'C' │ ╰─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────╯

georgesittas · 2026-07-01T12:57:27Z

@@ -962,11 +986,16 @@ def qualify_outputs(scope_or_expression: Scope | exp.Expr) -> None:
            if not selection.output_name:
                selection.set("alias", exp.TableAlias(this=exp.to_identifier(f"_col_{i}")))


What about this branch? This will still emit a lowercase _col_i in Snowflake. Is it normalized before quote_identifiers kicks in?

Lol, copilot made the same comment.

You're (both) right, it's been fixed and a test was added.

…schema

fivetran-kwoodbeck commented Jun 29, 2026

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georgesittas changed the title ~~fix(optimizer): normalization of synthesized output aliases~~ fix(optimizer)!: normalization of synthesized output aliases Jun 30, 2026

fivetran-kwoodbeck changed the title ~~fix(optimizer)!: normalization of synthesized output aliases~~ fix(optimizer): normalization of synthesized output aliases Jun 30, 2026

fivetran-kwoodbeck force-pushed the optimizer/bug-normalizealiases branch from 99348c9 to 9654789 Compare June 30, 2026 16:38

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georgesittas requested changes Jul 1, 2026

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georgesittas requested review from Copilot and tobymao July 1, 2026 12:50

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georgesittas changed the title ~~fix(optimizer): normalization of synthesized output aliases~~ fix(optimizer)!: normalization of synthesized output aliases Jul 1, 2026

fivetran-kwoodbeck added 8 commits July 3, 2026 11:04

Normalization of synthesized output aliases

96a6b13

PR tweaks / updates

314b105

tweak to deal with dialects that preserve case

762eb85

adjusted comment

b854e22

updated tests

a76206e

updated tests

44caa96

fixed test schema, made dialect non optional, and infers quotes from …

cf071d4

…schema

updated tests

eb6c093

fivetran-kwoodbeck force-pushed the optimizer/bug-normalizealiases branch from 478351a to eb6c093 Compare July 3, 2026 15:15

minor tweak

4bccdf6

fivetran-kwoodbeck added 4 commits July 3, 2026 12:53

fixed tests

13a847c

updated tests

e4f77f8

fixed other branch

1c765e1

formatting

3003df7

		def qualify_outputs(scope_or_expression: Scope \| exp.Expr) -> None:
		def qualify_outputs(scope_or_expression: Scope \| exp.Expr, dialect: Dialect \| None = None) -> None:

		@@ -962,11 +986,16 @@ def qualify_outputs(scope_or_expression: Scope \| exp.Expr) -> None:
		if not selection.output_name:
		selection.set("alias", exp.TableAlias(this=exp.to_identifier(f"_col_{i}")))

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fivetran-kwoodbeck commented Jun 29, 2026 •

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