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Query Optimization

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This chapter is from the book

This chapter is from the book

Loading Data Efficiently

Most of the time you'll probably be concerned about optimizing SELECT queries, because they are the most common type of query and because it's not always straightforward to figure out how to optimize them. By comparison, loading data into your database is straightforward. Nevertheless, there are strategies you can use to improve the efficiency of data-loading operations. The basic principles are these:

  • Bulk loading is more efficient than single-row loading because the key cache need not be flushed after each record is loaded; it can be flushed at the end of the batch of records. The more you can reduce key cache flushing, the faster data loading will be.

  • Loading is faster when a table has no indexes than when it is indexed. If there are indexes, not only must the record be added to the data file, but also each index must be modified to reflect the addition of the new record.

  • Shorter SQL statements are faster than longer statements because they involve less parsing on the part of the server and because they can be sent over the network from the client to the server more quickly.

Some of these factors may seem minor (the last one in particular), but if you're loading a lot of data, even small efficiencies make a difference. From the preceding general principles, several practical conclusions can be drawn about how to load data most quickly:

  • LOAD DATA (all forms) is more efficient than INSERT because it loads rows in bulk. The server must parse and interpret only one statement, not several. Also, the index needs flushing only after all rows have been processed, rather than after each row.

  • LOAD DATA is more efficient without LOCAL than with it. Without LOCAL, the file must be located on the server and you must have the FILE privilege, but the server can read the file directly from disk. With LOAD DATA LOCAL, the client reads the file and sends it over the network to the server, which is slower.

  • If you must use INSERT, try to use the form that allows multiple rows to be specified in a single statement:

  • INSERT INTO tbl_name VALUES(...),(...),... ;

    The more rows you can specify in the statement, the better. This reduces the total number of statements you need and minimizes the amount of index flushing. This might seem to contradict the earlier remark that shorter statements can be processed faster than longer statements. But there is no contradiction. The principles here are that a single INSERT statement that inserts multiple rows is shorter overall than an equivalent set of individual single-row INSERT statements, and the multiple-row statement can be processed on the server with much less index flushing.

    If you use mysqldump to generate database backup files, it generates multiple-row INSERT statements by default as of MySQL 4.1: The --opt (optimize) option is enabled, which turns on the --extended-insert option that produces multiple-row INSERT statements, as well as some other options that allow the dump file to be processed more efficiently when it is reloaded. Before MySQL 4.1, you can specify the --opt or --extended-insert option explicitly.

    Avoid using the --complete-insert option with mysqldump; the resulting INSERT statements will be for single rows and will be longer and require more parsing than will multiple-row statements.

  • If you must use multiple INSERT statements, group them if possible to reduce index flushing. For transactional storage engines, do this by issuing the INSERT statements within a single transaction rather than in autocommit mode:

  • START TRANSACTION;
    INSERT INTO tbl_name ... ;
    INSERT INTO tbl_name ... ;
    INSERT INTO tbl_name ... ;
    COMMIT;

    For non-transactional storage engines, obtain a write lock on the table and issue the INSERT statements while the table is locked:

    LOCK TABLES tbl_name WRITE;
    INSERT INTO tbl_name ... ;
    INSERT INTO tbl_name ... ;
    INSERT INTO tbl_name ... ;
    UNLOCK TABLES;

    Either way, you obtain the same benefit: The index is flushed once after all the statements have been executed rather than once per INSERT statement. The latter is what happens in autocommit mode or if the table has not been locked.

  • For MyISAM tables, another strategy for reducing index flushing is to use the DELAYED_KEY_WRITE table option. With this option, data rows are written to the data file immediately as usual, but the key cache is flushed only occasionally rather than after each insert. To use delayed index flushing on a server-wide basis, start mysqld with the --delay-key-write option. In this case, index block writes for a table are delayed until blocks must be flushed to make room for other index values, until a FLUSH TABLES command has been executed, or until the table is closed.

  • If you choose to use delayed key writes for MyISAM tables, abnormal server shutdowns can cause loss of index values. This is not a fatal problem because MyISAM indexes can be repaired based on the data rows, but to make sure that the repairs happen, you should start the server with the --myisam-recover=FORCE option. This option causes the server to check MyISAM tables when it opens them and repair them automatically if necessary.

    For a replication slave server, you might want to use --delay-key-write=ALL to delay index flushing for all MyISAM tables, regardless of how they were created originally on the master server.

  • Use the compressed client/server protocol to reduce the amount of data going over the network. For most MySQL clients, this can be specified using the --compress command-line option. Generally, this should only be used on slow networks because compression requires quite a bit of processor time.

  • Let MySQL insert default values for you. That is, don't specify columns in INSERT statements that will be assigned the default value anyway. On average, your statements will be shorter, reducing the number of characters sent over the network to the server. In addition, because the statements contain fewer values, the server does less parsing and value conversion.

  • For MyISAM tables, if you need to load a lot of data into a new table to populate it, it's faster to create the table without indexes, load the data, and then create the indexes. It's faster to create the indexes all at once rather than to modify them for each row. For a table that already has indexes, data loading may be faster if you drop or deactivate the indexes beforehand, and then rebuild or reactivate them afterward. These strategies do not apply to InnoDB or BDB tables, which do not have optimizations for separate index creation.

If you're considering using the strategy of dropping or deactivating indexes for loading data into MyISAM tables, think about the overall circumstances of your situation in assessing whether any benefit is likely to be obtained. If you're loading a small amount of data into a large table, rebuilding the indexes probably will take longer than just loading the data without any special preparation.

To drop and rebuild indexes, use DROP INDEX and CREATE INDEX, or the index-related forms of ALTER TABLE. To deactivate and reactivate indexes, you have two choices:

  • You can use the DISABLE KEYS and ENABLE KEYS forms of ALTER TABLE:

  • ALTER TABLE tbl_name DISABLE KEYS;
    ALTER TABLE tbl_name ENABLE KEYS;

    These statements turn off and on updating of any non-unique indexes in the table.

    The DISABLE KEYS and ENABLE KEYS clauses for ALTER TABLE are the preferred method for index deactivation and activation, because the server does the work. (Note that if you're using a LOAD DATA statement to load data into an empty MyISAM table, the server performs this optimization automatically.)

  • The myisamchk utility can perform index manipulation. It operates directly on the table files, so to use it you must have write access to the table files. You should also observe the precautions described in "Performing Database Maintenance with the Server Running," in Chapter 13, "Database Backups, Maintenance, and Repair," for keeping the server from accessing a table while you're using its files.

  • To deactivate a MyISAM table's indexes with myisamchk, first make sure you've told the server to leave the table alone, and then move into the appropriate database directory and run the following command:

    % myisamchk --keys-used=0 tbl_name

    After loading the table with data, reactivate the indexes:

    % myisamchk --recover --quick --keys-used=n tbl_name

    n is interpreted as a bitmask indicating which indexes to enable. Bit 0 (the first bit) corresponds to index 1. For example, if a table has three indexes, the value of n should be 7 (111 binary). You can determine index numbers with the --description option:

    % myisamchk --description tbl_name

The preceding data-loading principles also apply to mixed-query environments involving clients performing different kinds of operations. For example, you generally should avoid long-running SELECT queries on tables that are changed (written to) frequently. This causes a lot of contention and poor performance for the writers. A possible way around this, if your writes are mostly INSERT operations, is to add new records to an auxiliary table and then add those records to the main table periodically. This is not a viable strategy if you need to be able to access new records immediately, but if you can afford to leave them inaccessible for a short time, use of the auxiliary table will help you two ways. First, it reduces contention with SELECT queries that are taking place on the main table, so they execute more quickly. Second, it takes less time overall to load a batch of records from the auxiliary table into the main table than it would to load the records individually; the key cache need be flushed only at the end of each batch, rather than after each individual row.

One application for this strategy is when you're logging Web page accesses from your Web server into a MySQL database. In this case, it may not be a high priority to make sure that the entries get into the main table right away.

If you're using mixed INSERT and SELECT statements with a MyISAM table, you might be able to take advantage of concurrent inserts. This feature allows the inserts to take place at the same time as retrievals without the use of an auxiliary table. See "Using Concurrent Inserts" for details.

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