8.4.1 Optimizing Data Size
Design your tables to minimize their space on the disk. This can result in huge improvements by reducing the amount of data written to and read from disk. Smaller tables normally require less main memory while their contents are being actively processed during query execution. Any space reduction for table data also results in smaller indexes that can be processed faster.
MySQL supports many different storage engines (table types) and row formats. For each table, you can decide which storage and indexing method to use. Choosing the proper table format for your application can give you a big performance gain. See Chapter 14, The InnoDB Storage Engine, and Chapter 15, Alternative Storage Engines.
You can get better performance for a table and minimize storage space by using the techniques listed here:
Use the most efficient (smallest) data types possible. MySQL has many specialized types that save disk space and memory. For example, use the smaller integer types if possible to get smaller tables.
MEDIUMINTis often a better choice than
MEDIUMINTcolumn uses 25% less space.
Declare columns to be
NOT NULLif possible. It makes SQL operations faster, by enabling better use of indexes and eliminating overhead for testing whether each value is
NULL. You also save some storage space, one bit per column. If you really need
NULLvalues in your tables, use them. Just avoid the default setting that allows
NULLvalues in every column.
InnoDBtables are created using the
DYNAMICrow format by default. To use a row format other than
innodb_default_row_format, or specify the
ROW_FORMAToption explicitly in a
The compact family of row formats, which includes
COMPRESSED, decreases row storage space at the cost of increasing CPU use for some operations. If your workload is a typical one that is limited by cache hit rates and disk speed it is likely to be faster. If it is a rare case that is limited by CPU speed, it might be slower.
The compact family of row formats also optimizes
CHARcolumn storage when using a variable-length character set such as
N× the maximum byte length of the character set. Many languages can be written primarily using single-byte
utf8characters, so a fixed storage length often wastes space. With the compact family of rows formats,
InnoDBallocates a variable amount of storage in the range of
N× the maximum byte length of the character set for these columns by stripping trailing spaces. The minimum storage length is
Nbytes to facilitate in-place updates in typical cases. For more information, see Section 14.11, “InnoDB Row Formats”.
To minimize space even further by storing table data in compressed form, specify
InnoDBtables, or run the myisampack command on an existing
InnoDBcompressed tables are readable and writable, while
MyISAMcompressed tables are read-only.)
MyISAMtables, if you do not have any variable-length columns (
BLOBcolumns), a fixed-size row format is used. This is faster but may waste some space. See Section 15.2.3, “MyISAM Table Storage Formats”. You can hint that you want to have fixed length rows even if you have
VARCHARcolumns with the
The primary index of a table should be as short as possible. This makes identification of each row easy and efficient. For
InnoDBtables, the primary key columns are duplicated in each secondary index entry, so a short primary key saves considerable space if you have many secondary indexes.
Create only the indexes that you need to improve query performance. Indexes are good for retrieval, but slow down insert and update operations. If you access a table mostly by searching on a combination of columns, create a single composite index on them rather than a separate index for each column. The first part of the index should be the column most used. If you always use many columns when selecting from the table, the first column in the index should be the one with the most duplicates, to obtain better compression of the index.
If it is very likely that a long string column has a unique prefix on the first number of characters, it is better to index only this prefix, using MySQL's support for creating an index on the leftmost part of the column (see Section 13.1.14, “CREATE INDEX Statement”). Shorter indexes are faster, not only because they require less disk space, but because they also give you more hits in the index cache, and thus fewer disk seeks. See Section 5.1.1, “Configuring the Server”.
In some circumstances, it can be beneficial to split into two a table that is scanned very often. This is especially true if it is a dynamic-format table and it is possible to use a smaller static format table that can be used to find the relevant rows when scanning the table.
Declare columns with identical information in different tables with identical data types, to speed up joins based on the corresponding columns.
Keep column names simple, so that you can use the same name across different tables and simplify join queries. For example, in a table named
customer, use a column name of
customer_name. To make your names portable to other SQL servers, consider keeping them shorter than 18 characters.
Normally, try to keep all data nonredundant (observing what is referred to in database theory as third normal form). Instead of repeating lengthy values such as names and addresses, assign them unique IDs, repeat these IDs as needed across multiple smaller tables, and join the tables in queries by referencing the IDs in the join clause.
If speed is more important than disk space and the maintenance costs of keeping multiple copies of data, for example in a business intelligence scenario where you analyze all the data from large tables, you can relax the normalization rules, duplicating information or creating summary tables to gain more speed.