Updating column with unique constraint/index in Firebird in multiple rows - firebird

Question

I have a table that looks like this:
CREATE TABLE SCHEDULE (
RCDNO INTEGER NOT NULL,
MASTCONNO INTEGER NOT NULL,
ROWNO INTEGER DEFAULT 0 NOT NULL,
EDITTIMESTAMP TIMESTAMP
)
ALTER TABLE SCHEDULE ADD CONSTRAINT PK_SCHEDULE PRIMARY KEY (RCDNO);
CREATE UNIQUE INDEX IDX_SCHEDULE_3 ON SCHEDULE (MASTCONNO, ROWNO);
I can run a query like this:
select mastconno, rowno, rcdno, edittimestamp
from schedule
where mastconno = 12
order by rowno desc
and I get this:
Due to a bug in the app code, the time portion of the edittimestamp is missing but that's of no consequence here. The records are nonetheless listed in desc order of entry by ROWNO. That's what the design of this table is meant to facilitate.
What I tried to do is this...
update SCHEDULE
set ROWNO = (ROWNO + 1)
where MASTCONNO = 12
... in preparation for the insert of a new ROWNO=0 record, I get this error:
attempt to store duplicate value (visible to active transactions) in unique index "IDX_SCHEDULE_3".
Problematic key value is ("MASTCONNO" = 12, "ROWNO" = 3).
Incidently I have an exact copy of the table in MS SQL Server, and I didn't have this problem there. This seems to be specific to the way Firebird works.
So then I tried this, hoping that Firebird would "feed" values from the IN predicate to the UPDATE in a non-offending order.
update SCHEDULE
set ROWNO = (ROWNO + 1)
where MASTCONNO = 12 and
ROWNO in (select ROWNO from SCHEDULE
where MASTCONNO = 12 order by ROWNO DESC)
Sadly the response was the same as before. ROWNO 3 is being duplicated by the update statement.

Answer 1

With the unique index (MASTCONNO, ROWNO) in place, I have tested the following:
update SCHEDULE
set ROWNO = (ROWNO + 1)
where MASTCONNO = 12
order by ROWNO DESC
Works correctly!
update SCHEDULE
set ROWNO = (ROWNO - 1)
where MASTCONNO = 12
order by ROWNO ASC
Also works correctly!
Thanks very much Mark Rotteveel.

Answer 2

Indeed, constraint checks and triggers Firebird runs per every row, not per transaction or per table.
So you would have to use the same loop-based approach you use in applications dealing with lists and arrays.
You have to use EXECUTE BLOCK and a properly directed loop.
http://www.firebirdsql.org/file/documentation/reference_manuals/fblangref25-en/html/fblangref25-dml-execblock.html
You take MAXimum ID that is of interest, and increment it.
Then you take previous value.
Then previous...
EXECUTE BLOCK
AS
DECLARE ID INTEGER;
BEGIN
ID = ( SELECT MAX(ROWNO) FROM SCHEDULE WHERE MASTCONNO = 12 );
/*
ID = NULL;
SELECT MAX(ROWNO) FROM SCHEDULE WHERE MASTCONNO = 12 INTO :ID;
IF (ID IS NULL) raise-some-error-somehow
*/
While (ID >= 0)
Begin
update SCHEDULE set ROWNO = (ID + 1)
where MASTCONNO = 12 and :ID = ROWNO;
ID = ID - 1;
End
END
DANGER!
If while you started are doing it some another transaction (maybe from another program working at another computer) inserts some new row with MASTCONNO = 12 and commits it - you have problems.
Firebird is multi-versions server, not table-blocking server, so there will be nothing in server, that prohibits this insert because your procedure is working on the table. Then you have race conditions, the fastest transaction would commit itself, and the slower one would fail with unique index violation.
You may also use FOR-SELECT loop instead of WHILE loop.
https://www.firebirdsql.org/file/documentation/reference_manuals/fblangref25-en/html/fblangref25-psql-coding.html#fblangref25-psql-forselect
Like this
EXECUTE BLOCK
AS
BEGIN
FOR
SELECT * FROM SCHEDULE
WHERE MASTCONNO = 12
ORDER BY ROWNO DESCENDING -- proper loop direction: ordering is key!
AS CURSOR PTR
DO
UPDATE SCHEDULE SET ROWNO = ROWNO + 1
WHERE CURRENT OF PTR;
END
However, in Firebird 3 cursors-based positioning became rather slow.