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Ticket UUID: 892fc34f173e99d8aa03d52fbf42e0f9f2dfb210
Title: Incorrect query result when a LEFT JOIN subquery is flattened
Status: Fixed Type: Code_Defect
Severity: Severe Priority: Immediate
Subsystem: Unknown Resolution: Fixed
Last Modified: 2017-06-20 18:13:44
Version Found In: 3.19.3
User Comments:
drh added on 2017-06-20 16:15:03: (text/x-fossil-wiki)
The following SQL returns a different result in SQLite version 3.19.3 compared 
to SQLite version 3.18.0 and PostgreSQL.  The result is also different if
automatic indexes are disabled using "PRAGMA automatic_index=OFF".

<blockquote><verbatim>
CREATE TABLE t1(id INTEGER PRIMARY KEY);
CREATE TABLE t2(id INTEGER PRIMARY KEY, c2 INTEGER);
CREATE TABLE t3(id INTEGER PRIMARY KEY, c3 INTEGER);
INSERT INTO t1(id) VALUES(456);
INSERT INTO t3(id) VALUES(1),(2);

SELECT t1.id, x2.id, x3.id
FROM t1
LEFT JOIN (SELECT * FROM t2) AS x2 ON t1.id=x2.c2
LEFT JOIN t3 AS x3 ON x2.id=x3.c3;
</verbatim></blockquote>

This problem was introduced by the LEFT JOIN subquery flattener optimization
of check-in [41c27bc0ff] (which is also the cause of other bugs such as
[cad1ab4cb7b0] and [7fde638e94287d]).  The problem first appeared in the
3.19.0 release (2017-05-22).

This problem was reported on the SQLite users mailing list by Eric Sink.