SQLite

The DELETE operation on the penultimate line of the script below fails to delete every row for which b==2, as shown by the SELECT on the last line:

PRAGMA page_size=1024;
CREATE TABLE t1(a INTEGER PRIMARY KEY, b, c);
CREATE INDEX x1 ON t1(b, c);
INSERT INTO t1(a,b,c) VALUES(1, 1, zeroblob(80));
INSERT INTO t1(a,b,c) SELECT a+1, 1, c FROM t1;
INSERT INTO t1(a,b,c) SELECT a+2, 1, c FROM t1;
INSERT INTO t1(a,b,c) SELECT a+10, 2, c FROM t1 WHERE b=1;
INSERT INTO t1(a,b,c) SELECT a+20, 3, c FROM t1 WHERE b=1;
PRAGMA reverse_unordered_selects = ON;
DELETE FROM t1 WHERE b=2;
SELECT a FROM t1 WHERE b=2;

The underlying cause of this malfunction appears to be a defect in the OP_Prev opcode of the VDBE. However, we have so far been unable to cause incorrect behavior except by using reverse_unordered_selects and the multi-row on-pass delete optimization that was introduced in version 3.9.0 (check-in [8b93cc5937]).