Ticket Hash: | d6b36be38a06c0525731e6357125e6a35ecc4b79 | |||
Title: | Sub-select in LIMIT clause may access db without locks. | |||
Status: | Closed | Type: | Code_Defect | |
Severity: | Critical | Priority: | Immediate | |
Subsystem: | Code_Generator | Resolution: | Fixed | |
Last Modified: | 2012-12-07 19:41:22 | |||
Version Found In: | trunk | |||
Description: | ||||
This problem affects a SELECT statement only if all the following are true:
When SQLite processes a SELECT statement for which the FROM clause contains a single sub-select that is not eligible for the flattening optimization, it creates a co-routine that returns one row each time it is invoked to implement the sub-select. The vdbe code is:
Because of the way the SQL compiler works, the VerifyCookie routine is currently being invoked the first time the co-routine is invoked, not by the start of the main routine. If the main routine attempts to access the database before the co-routine is invoked, then it is an error. This may occur if either the LIMIT or the OFFSET expression of the main SELECT are themselves sub-selects that access the database. For example: CREATE TABLE abc(a, b, c); CREATE TABLE def(a, b, c); SELECT * FROM (SELECT * FROM def LIMIT 1 OFFSET 1) LIMIT (SELECT 20 FROM abc) |