Oracle Sequence以2而不是1开头

Question

Unexpected behavior:

I am encountering strange behavior of Oracle sequences with 11g (works with 10g):

CREATE SEQUENCE test_sequence START WITH 1;
CREATE TABLE test_table ( val INT );

INSERT INTO test_table VALUES ( test_sequence.NEXTVAL );

Even though the sequence starts with 1 , the first value inserted is 2 :

SELECT * FROM test_table;

       VAL
----------
         2

---

Expected behavior:

Selecting NEXTVAL without the insert works as expected:

CREATE SEQUENCE test_sequence_2 START WITH 1;

SELECT test_sequence_2.NEXTVAL FROM dual

   NEXTVAL
----------
         1

---

Question:

Can anyone reproduce this using Oracle 11g? Is this a known issue?

I'm using
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production .

Answer 1

This is documented in the 11.2 SQL Language Reference where it says,

If you attempt to insert a sequence value into a table that uses deferred segment creation, the first value that the sequence returns will be skipped.

See the link in Jeffrey Kemp's answer for a My Oracle Support (Metalink) note and a workaround.

Answer 2

I'd say the cause is this "undocumented feature". See My Oracle Support Document ID 1273858.1 (which is unfortunately behind a paywall and cannot be copied here).

Try it without deferred segment creation and see if the problem persists.

Answer 3

I can't reproduce on 11G, ie the table contains a 1 after following your steps.

However, it is debatable whether this should be considered an "issue", because sequences are never guaranteed to be gap-free. What START WITH guarantees is that the sequence will never return a value lower than the specified starting value - eg to avoid conflicts with existing data. I do agree however that what you are seeing is surprising and I would be interested to know the reason!

Answer 4

Use:

CREATE SEQUENCE SQ_SEQUENCE_NAME
    INCREMENT BY 1
    START WITH 1
    MINVALUE 0  -- This will ensure start at 1!
    MAXVALUE 99
    NOCYCLE
    NOCACHE
    ORDER;