Question

我试图了解nextval如何递增序列：在下面看来，在一个INSERT INTO ... VALUES声明中， nextval将增加两倍的序列。但仅限于第一个这样的陈述。我在ORACLE manual

中不理解这种行为的原因

CREATE TABLE test_table(a INT);
CREATE SEQUENCE seqa START WITH 10;
CREATE SEQUENCE seqb START WITH 10;

INSERT INTO test_table VALUES(seqb.NEXTVAL);
INSERT INTO test_table VALUES(seqb.NEXTVAL);
SELECT * FROM test_table;

结果test_table：

A
-----
11
12

，而：

SELECT seqa.NEXTVAL FROM DUAL; -- returns 10

Answer 1

这对我来说并不会发生（在Oracle 12C上）：

SQL> CREATE TABLE test_table(a INT);

Table created.

SQL> CREATE SEQUENCE seqa START WITH 10;

Sequence created.

SQL> CREATE SEQUENCE seqb START WITH 10;

Sequence created.

SQL> INSERT INTO test_table VALUES(seqb.NEXTVAL);

1 row created.

SQL> INSERT INTO test_table VALUES(seqb.NEXTVAL);

1 row created.

SQL> SELECT * FROM test_table;

         A
----------
        10
        11

Answer 2

注意我没有意识到问题有11g标签，我的答案是针对12c的。可能对已经升级或升级到12c的人有用。

除了@Tony的回答，在Oracle 12c上，您不需要明确的sequence。利用版本IDENTITY COLUMNS

中引入的12.1.

例如，


    SQL> CREATE TABLE new_identity_table
      2    (
      3      ID   NUMBER GENERATED ALWAYS AS IDENTITY,
      4      text VARCHAR2(50)
      5    );

    Table created.

    SQL>
    SQL> INSERT
      2  INTO new_identity_table
      3    (
      4      text
      5    )
      6    VALUES
      7    (
      8      'This table has an identity column'
      9    );

    1 row created.

    SQL> column text format A40;
    SQL>
    SQL> select * from new_identity_table;

            ID TEXT
    ---------- ----------------------------------------
             1 This table has an identity column

    SQL>

Oracle creates a `sequence` to populate the `identity column`. You can find it named as `ISEQ$$`

    SQL> select sequence_name, min_value, max_value, increment_by from user_sequences;

    SEQUENCE_NAME         MIN_VALUE  MAX_VALUE                     INCREMENT_BY
    -------------------- ---------- ----------------------------   ------------
    ISEQ$$_93199                  1 9999999999999999999999999999   1

    SQL>

More more information about the identity columns, use the `ALL_TAB_IDENTITY_COLS` view.

    SQL> SELECT table_name,
      2         column_name,
      3         generation_type,
      4         identity_options
      5  FROM   all_tab_identity_cols
      6  WHERE  owner = 'LALIT'
      7  ORDER BY 1, 2;

    TABLE_NAME           COLUMN_NAME     GENERATION IDENTITY_OPTIONS
    -------------------- --------------- ---------- --------------------------------------------------
    NEW_IDENTITY_TABLE   ID              ALWAYS     START WITH: 1, INCREMENT BY: 1, MAX_VALUE: 9999999
                                                    999999999999999999999, MIN_VALUE: 1, CYCLE_FLAG: N
                                                    , CACHE_SIZE: 20, ORDER_FLAG: N


    SQL>

oracle中的sequence.nextval似乎增加了两次

2 个答案: