mirror of https://github.com/citusdata/citus.git
a4040ba5da
This PR fixes #7784 and refactors the `WrapSubquery(Query *subquery)` function to improve clarity and correctness when handling volatile expressions in subqueries during Citus insert-select rewriting. ### Background The `WrapSubquery` function rewrites a query of the form: ```sql INSERT INTO target_table SELECT ... FROM ... ``` ...by wrapping the `SELECT` in a subquery: ```sql SELECT <outer-TL> FROM ( <subquery with volatile expressions replaced with NULL> ) citus_insert_select_subquery ``` This transformation allows: * **Volatile expressions** (e.g., `nextval`, `now`) **not used in `GROUP BY` or `ORDER BY`** to be evaluated **exactly once on the coordinator**. * **Stable/immutable or sort-relevant expressions** to remain in the worker-executed subquery. * Placeholder `NULL`s to maintain column alignment in the inner subquery. ### Fix Details * Restructured the code into labeled logical sections: 1. Build wrapper query (`SELECT … FROM (subquery)`) 2. Rewrite target lists with volatility analysis 3. Assign and return updated query trees * Preserved existing behavior, focusing on clarity and maintainability. ### How the new code handles volatile items stage | what we look for | what we do | why -- | -- | -- | -- scan target list once | 1. `expr_is_volatile(te->expr)` 2. `te->ressortgroupref != 0` (is the column used in GROUP BY / ORDER BY?) | decide whether to hoist or keep | we must not hoist an expression the inner query still needs for sorting/grouping, otherwise its `SortGroupClause` breaks volatile & not used in sort/group | deep‑copy the expression into the outer target list | executes once on the coordinator | | leave a typed `NULL `placeholder (visible, not `resjunk`) in the inner target list | keeps column numbering stable for helpers that already ran (reorder, cast); the worker sends a cheap constant | stable / immutable, or volatile but used in sort/group | keep the original expression in the inner list; outer list references it via a `Var `| workers can evaluate it safely and, if needed, the inner ORDER BY still works | ### Example Given this query: ```sql INSERT INTO t SELECT nextval('s'), 42 FROM generate_series(1, 2); ``` The planner rewrites it as: ```sql SELECT nextval('s'), col2 FROM (SELECT NULL::bigint AS col1, 42 AS col2 FROM generate_series(1, 2)) citus_insert_select_subquery; ``` This ensures `nextval('s')` is evaluated only once per row on the **coordinator**, not on each worker node, preserving correct sequence semantics. #### **Outer‑Var guard (`FindReferencedTableColumn`)** Because `WrapSubquery` adds an extra query level, lots of Vars that the old code never expected become “outer” Vars; without teaching `FindReferencedTableColumn` to climb that extra level reliably, Citus would intermittently reject valid foreign keys and even hit asserts. * Re‑implemented the outer‑Var guard so that the function: * **Walks deterministically up the query stack** when `skipOuterVars = false` (default for FK / UNION checks). A new while‑loop copies — rather than truncates — `parentQueryList` on each hop, eliminating list‑aliasing that made *issue 5248* fail intermittently in parallel regressions. * Handles multi‑level `varlevelsup` in a single loop; never mutates the caller’s list in place. |
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