Bug #119442: Fix ORDER BY optimization doesn't recognize IS NULL as constant #637
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PROBLEM: 1. The optimizer's check_field_is_const() function only recognized equality operators (col = value) as constants for ORDER BY simplification. 2. It failed to recognize that "col IS NULL" also guarantees a single value (NULL) for sorting purposes. 3. This caused unnecessary filesort operations even when covering indexes existed. 4. Example: WHERE a = 1 AND b IS NULL ORDER BY a, b, c would use filesort despite having index (a, b, c). FIX: 1. Extended check_field_is_const() to handle Item_func::ISNULL_FUNC. 2. Added is_isnull_func() helper function to identify IS NULL conditions. 3. Properly handles OR conditions - "IS NULL OR value" remains non-constant, while "IS NULL OR IS NULL" is constant. 4. Stores IS NULL function as sentinel in const_item to ensure consistency across OR branches. 5. Preserves correct NULL ordering semantics (NULLs first in ASC, last in DESC).
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What changes are proposed in this pull request?
Why?
This PR addresses Bug#119442. The MySQL optimizer currently fails to treat columns filtered by
IS NULLas "constants" duringORDER BYsimplification. While an equality predicate (e.g.,col = 5) allows the optimizer to skip sorting on that column,col IS NULLwhich functionally also guarantees a single value for sorting purposes is ignored.This limitation forces the engine to perform unnecessary
filesortoperations even when a covering index exists that could satisfy the sort order if theIS NULLcolumn were treated as a constant.What?
This change updates the optimizer's
check_field_is_const()logic to recognizeItem_func::ISNULL_FUNCas a valid constant condition. By treatingIS NULLas a constant, the optimizer can now rely on the index order for subsequent columns in theORDER BYclause, eliminatingfilesortin these scenarios.How?
check_field_is_const: Modified this function insql/sql_select.ccto inspectItem_func::ISNULL_FUNCalongside the existing equality checks.IS NULLdoes not have a literal "value" like an integer or string, theIS NULLfunction pointer itself is stored as a sentinel inconst_itemto track state across recursive calls (e.g., insideORconditions).ORbranches:... AND (col IS NULL OR col IS NULL)-> Recognized as Constant.... AND (col IS NULL OR col = 5)-> Recognized as Non-Constantis_isnull_func()to clean up some code.How is this tested?
New MTR Tests (
mysql-test/t/order_by_isnull.test)The suite covers the following scenarios:
WHERE col IS NULL ORDER BY col, next_coluses the index and removesUsing filesort.IS NULLallow the index to provide ordering for subsequent columns.IS NULL OR IS NULLis optimized, whileIS NULL OR col=1is correctly treated as non-constant.NULLsorting order (first inASC, last inDESC) is preserved.INFORMATION_SCHEMA.OPTIMIZER_TRACEthat the column is now flagged with"equals_constant_in_where": true.Existing Regression Tests
Updated
mysql-test/r/order_by_all.resultandmysql-test/r/order_by_none.result. These tests previously expectedUsing filesortforIS NULLqueries; they have been updated to reflect the improved execution plan (Using index).