Variable-length columns of Disk Data tables take up a fixed amount of space. For each row, this is equal to the space required to store the largest possible value for that column.

For general information about calculating these values, see Section 10.5, “Data Type Storage Requirements”.

You can obtain an estimate the amount of space available in data files and undo log files by querying the INFORMATION_SCHEMA.FILES table. For more information and examples, see Section 20.21, “The INFORMATION_SCHEMA FILES Table”.

In a Disk Data table, the first 256 bytes of a TEXT or BLOB column are stored in memory; only the remainder is stored on disk.

Each row in a Disk Data table uses 8 bytes in memory to point to the data stored on disk. This means that, in some cases, converting an in-memory column to the disk-based format can actually result in greater memory usage. For example, convering a CHAR(4) column from memory-based to disk-based format increases the amount of DataMemory used per row from 4 to 8 bytes.