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Ticket UUID: ce68383bf6aba2f8e2b0344d067364e22daf59dc
Title: Incorrect LEFT JOIN result with IS operator in WHERE clause and auto-index
Status: Fixed Type: Code_Defect
Severity: Important Priority: Immediate
Subsystem: Unknown Resolution: Fixed
Last Modified: 2017-07-21 19:33:16
Version Found In: 3.19.3
User Comments:
drh added on 2017-07-18 16:57:15: (text/x-fossil-wiki)
The same LEFT JOIN query gives different results depending on whether or
not automatic indexes are enabled.  The code below illustrates:

<blockquote><verbatim>
CREATE TABLE t1(w int);
CREATE TABLE t2(y int);
INSERT INTO t1 VALUES(NULL);
INSERT INTO t2 VALUES(1);
CREATE INDEX t2y ON t2(y);
SELECT * FROM t1 LEFT JOIN t2 WHERE t2.y IS t1.w;
PRAGMA automatic_index=off;
SELECT * FROM t1 LEFT JOIN t2 WHERE t2.y IS t1.w;
</verbatim></blockquote>

Both queries should return no rows.  But when automatic indexes are turned on
(the default) a single row of two NULLs is returned.

Bisecting shows this problem was introduced by check-in [3428043cd0029445]

This problem was discovered during internal testing and analysis and has
never been reported in the wild.