fix(duckdb,postgres,redshift,snowflake)!: ARRAY_CAT null propagation

[fivetran-MichaelLee]

Summary

Support for ARRAY_CAT null propagation in dialects:

• snowflake
• duckdb
• redshift
• postgres

Bug Fix

DuckDB's LIST_CONCAT function accepts variadic arguments but this was not recognized by sqlglot. Removed ARRAY_CONCAT_IS_VAR_LEN = False from the dialect and updated tests accordingly.

Edge Case Not Handled

Postgres ignores NULL arguments to ARRAY_CAT, but when all arguments are NULL it will return NULL. When transpiling to a null propagating dialect, the resulting sql will COALESCE all the arguments to an empty array. Handling this edge case would add to an already complex solution.

[github-actions]

SQLGlot Integration Test Results

Comparing:

• this branch (sqlglot:mlee/transpile-array_cat, sqlglot version: mlee/transpile-array_cat)
• baseline (main, sqlglot version: 28.6.1.dev77)

⚠️ Limited to dialects: snowflake, bigquery, duckdb

By Dialect

dialect	main	sqlglot:mlee/transpile-array_cat	difference	links
bigquery -> bigquery	2592/2624 passed (98.8%)	2592/2624 passed (98.8%)	No change	full result / delta
bigquery -> duckdb	1846/2623 passed (70.4%)	1846/2623 passed (70.4%)	No change	full result / delta
duckdb -> duckdb	4003/4003 passed (100.0%)	4003/4003 passed (100.0%)	No change	full result / delta
snowflake -> duckdb	615/847 passed (72.6%)	615/847 passed (72.6%)	No change	full result / delta
snowflake -> snowflake	847/847 passed (100.0%)	847/847 passed (100.0%)	No change	full result / delta

Overall

main: 10944 total, 9903 passed (pass rate: 90.5%), sqlglot version: 28.6.1.dev77

sqlglot:mlee/transpile-array_cat: 10944 total, 9903 passed (pass rate: 90.5%), sqlglot version: mlee/transpile-array_cat

Difference: No change

[georgesittas]

FYI @fivetran-MichaelLee CI's failing for formatting-related reasons. Drop a ping when ready.

[fivetran-MichaelLee]

@georgesittas This is going to need some more work so I'm keeping in draft for now