docs: add guidance on disabling constant folding for literal tests#3200
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comphead merged 1 commit intoapache:mainfrom Jan 16, 2026
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When testing expressions with literal values, Spark's constant folding optimizer may evaluate the expression at planning time, bypassing the Comet implementation entirely. This adds documentation on how to disable constant folding to ensure literal tests actually exercise Comet code. Co-Authored-By: Claude Opus 4.5 <[email protected]>
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Thanks @andygrove this good to note, we exclude it some tests like when testing literals and expressions but it was not documented in tests, but should really be
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Which issue does this PR close?
N/A - Documentation improvement
Rationale for this change
When testing expressions with literal values (e.g.,
SELECT my_func('literal')), Spark's constant folding optimizer may evaluate the expression at planning time rather than execution time. This means the Comet implementation might not actually be exercised during the test, leading to false confidence in test coverage.What changes are included in this PR?
Adds a new "Testing with Literal Values" section to the contributor guide for adding new expressions, explaining:
SQLConf.OPTIMIZER_EXCLUDED_RULESHow are these changes tested?
Documentation only - no code changes.