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. MEDIUMINT is often a better choice than INT because a MEDIUMINT column uses 25% less space.

Declare columns to be NOT NULL if possible. It makes everything faster and you save one bit per column. If you really need NULL in your application, you should definitely use it. Just avoid having it on all columns by default.

For MyISAM tables, if you do not have any variable-length columns (VARCHAR, TEXT, or BLOB columns), a fixed-size row format is used. This is faster but unfortunately may waste some space. See Section 14.1.3, “MyISAM Table Storage Formats”. You can hint that you want to have fixed length rows even if you have VARCHAR columns with the CREATE TABLE option ROW_FORMAT=FIXED.

InnoDB tables use a compact storage format. In versions of MySQL earlier than 5.0.3, InnoDB rows contain some redundant information, such as the number of columns and the length of each column, even for fixed-size columns. By default, tables are created in the compact format (ROW_FORMAT=COMPACT). If you wish to downgrade to older versions of MySQL, you can request the old format with ROW_FORMAT=REDUNDANT.

The compact InnoDB format also changes how CHAR columns containing UTF-8 data are stored. With ROW_FORMAT=REDUNDANT, a UTF-8 CHAR(N) occupies 3 × N bytes, given that the maximum length of a UTF-8 encoded character is three bytes. Many languages can be written primarily using single-byte UTF-8 characters, so a fixed storage length often wastes space. With ROW_FORMAT=COMPACT format, InnoDB allocates a variable amount of storage in the range from N to 3 × N bytes for these columns by stripping trailing spaces if necessary. The minimum storage length is kept as N bytes to facilitate in-place updates in typical cases.

The primary index of a table should be as short as possible. This makes identification of each row easy and efficient.

Create only the indexes that you really need. Indexes are good for retrieval but bad when you need to store data quickly. If you access a table mostly by searching on a combination of columns, create an index on them. The first part of the index should be the column most used. If you always use many columns when selecting from the table, you should use the column with more duplicates first to obtain better compression of the index.

If it is very likely that a string column has a unique prefix on the first number of characters, it's better to index only this prefix, using MySQL's support for creating an index on the leftmost part of the column (see Section 13.1.4, “CREATE INDEX Syntax”). Shorter indexes are faster, not only because they require less disk space, but because they give also you more hits in the index cache, and thus fewer disk seeks. See Section 7.5.2, “Tuning Server Parameters”.

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.