[DERBY-6494] NEXT VALUE FOR should be illegal inside a COALESCE expression

• Type: Bug
• Status: Open
• Priority: Minor
• Resolution: Unresolved
• Affects Version/s: 10.11.1.1
• Component/s: SQL
• Urgency: Low
• Issue & fix info: Repro attached
• Bug behavior facts: Deviation from standard, Seen in production
• Assignee: Unassigned
• Reporter: Richard N. Hillegas
• Created: 04/Mar/14 19:03
• Updated: 04/Mar/14 19:04

Description

It should be illegal to use a NEXT VALUE FOR expression as one of the values in a COALESCE expression. That is because the result of a NEXT VALUE FOR clause is non-deterministic. However, Derby allows this forbidden behavior, as the following script shows:

connect 'jdbc:derby:memory:db;create=true';

create sequence seq_cpd_code start with 0;

-- a COALESCE expression can invoke a sequence generator...
values coalesce( 99, next value for seq_cpd_code );

-- ...but a CASE expression cannot...
values case when 99 is not null then 99 else next value for seq_cpd_code end;

Here is how I reason that NEXT VALUE FOR should be forbidden in COALESCE expressions:

1) According to the 2011 SQL Standard, part 2, section 6.12 (case expression), syntax rule 1d...

COALESCE( v1, v2 )

...is supposed to be equivalent to...

CASE v1 IS NOT NULL
THEN v1
ELSE v2
END

2) However, a NEXT VALUE FOR expression may not appear in a CASE expression, according to part 2, section 6.14 (next value expression), syntax rule 2.

It appears that people are using NEXT VALUE FOR expressions inside COALESCE expressions (see http://apache-database.10148.n7.nabble.com/Unnecessary-increment-of-sequence-td136260.html). For that reason, I am not inclined to fix this defect.

Issue Links

• relates to: DERBY-712 Support for sequences