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13.2.3. HANDLER Syntax

HANDLER tbl_name OPEN [ AS alias ]
HANDLER tbl_name READ index_name { = | >= | <= | < } (value1,value2,...)
    [ WHERE where_condition ] [LIMIT ... ]
HANDLER tbl_name READ index_name { FIRST | NEXT | PREV | LAST }
    [ WHERE where_condition ] [LIMIT ... ]
HANDLER tbl_name READ { FIRST | NEXT }
    [ WHERE where_condition ] [LIMIT ... ]
HANDLER tbl_name CLOSE

The HANDLER statement provides direct access to table storage engine interfaces. It is available for MyISAM and InnoDB tables.

The HANDLER ... OPEN statement opens a table, making it accessible via subsequent HANDLER ... READ statements. This table object is not shared by other threads and is not closed until the thread calls HANDLER ... CLOSE or the thread terminates. If you open the table using an alias, further references to the open table with other HANDLER statements must use the alias rather than the table name.

The first HANDLER ... READ syntax fetches a row where the index specified satisfies the given values and the WHERE condition is met. If you have a multiple-column index, specify the index column values as a comma-separated list. Either specify values for all the columns in the index, or specify values for a leftmost prefix of the index columns. Suppose that an index my_idx includes three columns named col_a, col_b, and col_c, in that order. The HANDLER statement can specify values for all three columns in the index, or for the columns in a leftmost prefix. For example:

HANDLER ... READ my_idx = (col_a_val,col_b_val,col_c_val) ...
HANDLER ... READ my_idx = (col_a_val,col_b_val) ...
HANDLER ... READ my_idx = (col_a_val) ...

To employ the HANDLER interface to refer to a table's PRIMARY KEY, use the quoted identifier `PRIMARY`:

HANDLER tbl_name READ `PRIMARY` ...

The second HANDLER ... READ syntax fetches a row from the table in index order that matches the WHERE condition.

The third HANDLER ... READ syntax fetches a row from the table in natural row order that matches the WHERE condition. It is faster than HANDLER tbl_name READ index_name when a full table scan is desired. Natural row order is the order in which rows are stored in a MyISAM table data file. This statement works for InnoDB tables as well, but there is no such concept because there is no separate data file.

Without a LIMIT clause, all forms of HANDLER ... READ fetch a single row if one is available. To return a specific number of rows, include a LIMIT clause. It has the same syntax as for the SELECT statement. See Section 13.2.7, “SELECT Syntax”.

HANDLER ... CLOSE closes a table that was opened with HANDLER ... OPEN.

HANDLER is a somewhat low-level statement. For example, it does not provide consistency. That is, HANDLER ... OPEN does not take a snapshot of the table, and does not lock the table. This means that after a HANDLER ... OPEN statement is issued, table data can be modified (by the current thread or other threads) and these modifications might be only partially visible to HANDLER ... NEXT or HANDLER ... PREV scans.

There are several reasons to use the HANDLER interface instead of normal SELECT statements:

  • HANDLER is faster than SELECT:

    • A designated storage engine handler object is allocated for the HANDLER ... OPEN. The object is reused for subsequent HANDLER statements for that table; it need not be reinitialized for each one.

    • There is less parsing involved.

    • There is no optimizer or query-checking overhead.

    • The table does not have to be locked between two handler requests.

    • The handler interface does not have to provide a consistent look of the data (for example, dirty reads are allowed), so the storage engine can use optimizations that SELECT does not normally allow.

  • For applications that use a low-level ISAM-like interface, HANDLER makes it much easier to port them to MySQL.

  • HANDLER enables you to traverse a database in a manner that is difficult (or even impossible) to accomplish with SELECT. The HANDLER interface is a more natural way to look at data when working with applications that provide an interactive user interface to the database.


 
 
  Published under the terms of the GNU General Public License Design by Interspire