Implement deterministic RAND(N) and improve unseeded RAND() translation#341
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Implement deterministic RAND(N) and improve unseeded RAND() translation#341wp-fuse wants to merge 2 commits into
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This was referenced Apr 17, 2026
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Thanks for drafting this! I went deeper into a review and realized that it needs multiple changes, so I created #363. Most notably, it needed extensive test coverage, and there were unhandled cases that I addressed in that PR. |
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## Summary Replaces the old `mt_rand(0, 1)` stub — which returned an integer 0 or 1, not a float in `[0, 1)` as MySQL requires — with MySQL-compatible `RAND()` behavior. Replaces #341. ## Unseeded `RAND()` Compiles to a native SQLite expression: ```sql ((RANDOM() & 0x001FFFFFFFFFFFFF) / 9007199254740992.0) ``` The 53-bit mask matches the IEEE 754 double mantissa, so division by 2^53 is exact and strictly less than 1.0. Matches MySQL, where unseeded `RAND()` uses a thread-level state independent of `RAND(N)`. ## Seeded `RAND(N)` Routes through a PHP UDF implementing MySQL's exact LCG from `my_rnd_init()` / `my_rnd()` (`sql/item_func.cc`, `mysys/my_rnd.cc`), bit-exact against MySQL 9.6. Requires 64-bit PHP. Seed handling matches `val_int()`: `NULL` becomes 0, floats round to nearest (`RAND(3.9) == RAND(4)`), numeric strings follow the same path. ## Known divergences - MySQL distinguishes constant vs non-constant seeds at parse time (constant = init once per statement, non-constant = reinit per row). A SQLite UDF can't see the expression, so we approximate by reinitializing only when the seed value changes. This diverges when a non-constant expression yields a stable value. - The UDF keeps one LCG state per connection, so multiple `RAND(N)` call sites in one query share a stream: `SELECT RAND(1), RAND(1)` returns `(v1, v2)` here vs `(v, v)` in MySQL. Both are documented in the `rand()` docblock. ## Metadata `RAND()` column metadata is now reported as `DOUBLE` / `PARAM_STR`, removing a long-standing TODO. ## Test plan - [ ] CI green. - [ ] Bit-exact reference values against MySQL 9.6 (seeds 0, 1, 3, 5, and multi-row sequences). - [ ] NULL, float, numeric/non-numeric string, and negative seed handling. - [ ] `RAND()` in `WHERE`, `UPDATE`, `INSERT`, `ORDER BY` (`LIMIT` + seeded deterministic permutation). - [ ] Per-statement flush contract inside a transaction.
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Summary
This PR improves the MySQL-compatible
RAND()support in the AST-based SQLite driver. It replaces the previous generic implementation with a bit-exact MySQL Linear Congruential Generator (LCG) and a robust SQLite-level translation for unseeded calls.Technical Changes
rand()UDF inWP_SQLite_PDO_User_Defined_Functionsto use an instance-local LCG.0x10001,55555555,0x10000001) to ensure bit-exact deterministic output for given seeds, as requested in the previous review.mt_srand()andmt_rand()usage to prevent PHP global state pollution.WP_PDO_MySQL_On_SQLiteto use:((RANDOM() & 0x7FFFFFFFFFFFFFFF) / 9223372036854775808.0).ABS(-9223372036854775808)in SQLite and ensures an unbiased distribution in the[0, 1)range.RAND()fix from other compatibility layers. All string-replacement hacks and non-AST layers have been excluded.Validation
phpcs.