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25.39.30 The INFORMATION_SCHEMA INNODB_VIRTUAL Table

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The INNODB_VIRTUAL table provides metadata about InnoDB virtual generated columns and columns upon which virtual generated columns are based.

A row appears in the INNODB_VIRTUAL table for each column upon which a virtual generated column is based.

The INNODB_VIRTUAL table has these columns:

  • TABLE_ID

    An identifier representing the table associated with the virtual column; the same value as INNODB_TABLES.TABLE_ID.

  • POS

    The position value of the virtual generated column. The value is large because it encodes the column sequence number and ordinal position. The formula used to calculate the value uses a bitwise operation: 

    ((nth virtual generated column for the InnoDB instance + 1) << 16)
+ the ordinal position of the virtual generated column

    For example, if the first virtual generated column in the InnoDB instance is the third column of the table, the formula is (0 + 1) << 16) + 2. The first virtual generated column in the InnoDB instance is always number 0. As the third column in the table, the ordinal position of the virtual generated column is 2. Ordinal positions are counted from 0.

  • BASE_POS

    The ordinal position of the columns upon which a virtual generated column is based.

Example

  1. mysql> CREATE TABLE `t1` (
  2.          `a` int(11) DEFAULT NULL,
  3.          `b` int(11) DEFAULT NULL,
  4.          `c` int(11) GENERATED ALWAYS AS (a+b) VIRTUAL,
  5.          `h` varchar(10) DEFAULT NULL
  6.        ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
  7.  
  8. mysql> SELECT * FROM INFORMATION_SCHEMA.INNODB_VIRTUAL
  9.        WHERE TABLE_ID IN
  10.          (SELECT TABLE_ID FROM INFORMATION_SCHEMA.INNODB_TABLES
  11.           WHERE NAME LIKE "test/t1");
  12. +----------+-------+----------+
  13. | TABLE_ID | POS   | BASE_POS |
  14. +----------+-------+----------+
  15. |       98 | 65538 |        0 |
  16. |       98 | 65538 |        1 |
  17. +----------+-------+----------+

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Notes

  • If a constant value is assigned to a virtual generated column, as in the following table, an entry for the column does not appear in the INNODB_VIRTUAL table. For an entry to appear, a virtual generated column must have a base column.

    1. CREATE TABLE `t1` (
    2.   `a` int(11) DEFAULT NULL,
    3.   `b` int(11) DEFAULT NULL,
    4.   `c` int(11) GENERATED ALWAYS AS (5) VIRTUAL
    5. ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

    However, metadata for such a column does appear in the INNODB_COLUMNS table.

  • You must have the PROCESS privilege to query this table.

  • Use the INFORMATION_SCHEMA COLUMNS table or the SHOW COLUMNS statement to view additional information about the columns of this table, including data types and default values.


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Document created the 26/06/2006, last modified the 26/10/2018
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References

  1. View the html document Language of the document:en Manuel MySQL : https://dev.mysql.com/

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