SNOW-3176017: Fix accidental removal of aliases in certain JOIN statements

[sfc-gh-joshi]

1. Which Jira issue is this PR addressing? Make sure that there is an accompanying issue to your PR.

   Fixes SNOW-3176017

2. Fill out the following pre-review checklist:

   • I am adding a new automated test(s) to verify correctness of my new code
     • If this test skips Local Testing mode, I'm requesting review from @snowflakedb/local-testing
   • I am adding new logging messages
   • I am adding a new telemetry message
   • I am adding new credentials
   • I am adding a new dependency
   • If this is a new feature/behavior, I'm adding the Local Testing parity changes.
   • I acknowledge that I have ensured my changes to be thread-safe. Follow the link for more information: Thread-safe Developer Guidelines
   • If adding any arguments to public Snowpark APIs or creating new public Snowpark APIs, I acknowledge that I have ensured my changes include AST support. Follow the link for more information: AST Support Guidelines

3. Please describe how your code solves the related issue.

Reverts #4095, restoring the optimizations made in SNOW-2895675.

The original optimization, which replaced a layer of SELECT "A" AS "A", "B" AS "B" with SELECT * in certain join operations, caused a test failure in SnowML's CI pipeline. A minimal version of the test is reproduced in this PR in test_query_generator.py::test_disambiguate_skips_quoted_alias, and is as follows:

df1 = session.read.parquet(stage_filename)
df2 = session.create_dataframe(data, schema=["ID", "A", "B"])
df_res = df1.join(df2, on=["ID"])[['"COL_0"', '"COL_1"']]

This would generate SQL like the following:

bad SQL

SELECT 
    "COL_0", 
    "COL_1"
 FROM (
 SELECT  * 
 FROM (
(
 SELECT $1 AS "ID", $2 AS """COL_0""", $3 AS """COL_1""" FROM  VALUES (0 :: INT, 1 :: INT, 2 :: INT), (3 :: INT, 4 :: INT, 5 :: INT
)
) AS SNOWPARK_LEFT 
INNER JOIN 
(
 SELECT 
    "ID", 
    "A", 
    "B"
 FROM (
 SELECT $1 AS "ID", $2 AS "A", $3 AS "B" FROM  VALUES (0 :: INT, 1 :: INT, 2 :: INT), (3 :: INT, 4 :: INT, 5 :: INT)
)
) AS SNOWPARK_RIGHT
 USING (ID)
)
)

The file read operation was producing aliases of triple-quoted column names (which are used in SQL to produce single-quoted column names), which were silently dropped by the join optimization, resulting in the top-level references to COL_0 and COL_1 becoming invalid.

When _disambiguate is called during join operations, the new optimization skipped alias generation if the left and right side did not share any common column names. However, the _alias_if_needed actually serves the additional function of stripping quotes from identifiers it processes, including from triple-quoted identifiers. Even though there were no common column names in this case, aliasing was still necessary to rename """COL_0""" to "COL_0".

This error path is triggered only by ReadFile operations, which more or less directly generate their own SQL SELECT/COPY statements with aliasing inside. The join operation then implicitly normalized triple-quoted names like """COL_0""" to "COL_0" (whether this behavior is itself correct should be investigated separately).

[sfc-gh-yuwang]

some merge gates are still failing

[sfc-gh-aling]

can you help run the ML test job to verify that the change won't cause any error related to identifier compilation error?

[sfc-gh-joshi]

can you help run the ML test job to verify that the change won't cause any error related to identifier compilation error?

The affected test in the SnowML job appears to be failing for database permission reasons, but I manually verified that the issue is no longer present when running the test locally.

[graphite-app]

Telemetry is now sent unconditionally for all joins, even when there are no common column names requiring aliasing. Previously (line 333), this was conditional on if common_col_names:. This will cause telemetry spam for joins that don't actually need aliasing.

Fix: Restore the conditional check:

if common_col_names:
    # We use the session of the LHS DataFrame to report this telemetry
    lhs._session._conn._telemetry_client.send_alias_in_join_telemetry()

Suggested change

	# We use the session of the LHS DataFrame to report this telemetry
	lhs._session._conn._telemetry_client.send_alias_in_join_telemetry()
	if common_col_names:
	# We use the session of the LHS DataFrame to report this telemetry
	lhs._session._conn._telemetry_client.send_alias_in_join_telemetry()

Spotted by Graphite



Is this helpful? React 👍 or 👎 to let us know.