Stephan Szabo wrote:
On Tue, 2 Oct 2007, Jeff Frost wrote:

I expected these numbers to be in sync, but was suprised to see that the
sequence skips a values after every generate series.

CREATE TABLE jefftest ( id serial, num int );
INSERT INTO jefftest (num) values (generate_series(1,10));
INSERT INTO jefftest (num) values (generate_series(11,20));
INSERT INTO jefftest (num) values (generate_series(21,30));

It seems to do what you'd expect if you do
 INSERT INTO jefftest(num) select a from generate_series(1,10) as foo(a);
 INSERT INTO jefftest(num) select a from generate_series(11,20) as foo(a);
 INSERT INTO jefftest(num) select a from generate_series(21,30) as foo(a);

I tried a function that raises a notice and called it as
 select f1(1), generate_series(1,10);
and got 11 notices so it looks like there's some kind of phantom involved.

That's interesting - might need an answer from the core hackers.
I am posting this to pgsql-hackers to get their comments and feedback.
I wouldn't count it as a bug but it could be regarded as undesirable side effects.

My guess is that what appears to happen is that the sequence is created by incrementing as part of the insert steps and the test to check the end of the sequence is -
if last_inserted_number > end_sequence_number

This would explain the skip in sequence numbers.

My thoughts are that -
if last_inserted_number < end_sequence_number

would be a better way to approach this. Of course you would also need to check that the (last_insert + step_size) isn't greater than the end_sequence_number when the step_size is given.

I haven't looked at the code so I don't know if that fits easily into the flow of things.

The as foo(a) test would fit this as the sequence is generated into the equivalent of a temporary table the same as a subselect, then used as insert data. The rollback would be applied during the temporary table generation so won't show when the data is copied across to fulfill the insert.

Maybe the planner or the generate series function could use a temporary table to give the same results as select from generate_series()


Shane Ambler

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