BLOB considerations

A BLOB is an uninterpreted string of bytes or characters, stored as a value in a column. Common examples of a BLOB are picture or sound files. While BLOBs are typically large, you can store them in any character string or binary string data type such as CHAR, VARCHAR, NCHAR, BINARY, VARBINARY, and so on. Choose your data type and length depending on the content and length of BLOBs you expect to store.

Note

While a character large object is commonly called a CLOB, a binary large object is called a BLOB, and the combination of both is called a LOB. Only the acronym BLOB is used in this documentation.

When you create a column for storing BLOB values, you can control aspects of their storage. For example, you can specify that BLOBs up to a specified size be stored in the row (inline), while larger BLOBs are stored outside the row in table extension pages. Additionally, you can specify that for BLOBs stored outside the row, the first n bytes of the BLOB, also referred to as the prefix, are duplicated in the row. These storage aspects are controlled by the INLINE and PREFIX settings specified in the CREATE TABLE and ALTER TABLE statements. The values you specify for these settings can have unanticipated impacts on performance or disk storage requirements.

If neither INLINE nor PREFIX is specified, or if INLINE USE DEFAULT or PREFIX USE DEFAULT is specified, default values are applied as follows:

For character data type columns, such as CHAR, NCHAR, LONG VARCHAR, and XML, the default value of INLINE is 256, and the default value of PREFIX is 8.
For binary data type columns, such as BINARY, LONG BINARY, VARBINARY, BIT, VARBIT, LONG VARBIT, BIT VARYING, and UUID, the default value of INLINE is 256, and the default value of PREFIX is 0.

It is recommended that you do not set INLINE and PREFIX values unless there are specific requirements for which the default values are insufficient. The default values have been chosen to balance performance and disk space requirements. For example, row processing performance may degrade if you set INLINE to a large value, and all the BLOBs are stored inline. If you set PREFIX too high, you increase the amount of disk space required to store BLOBs since the prefix data duplicates a portion of the BLOB.

If you do decide to set INLINE or PREFIX values, the INLINE length must not exceed the length of the column. Likewise, the PREFIX length, must not exceed the INLINE length.

The prefix data for a compressed column is stored uncompressed, so if all the data required to satisfy a request is stored in the prefix, no decompression is necessary.

For information about the defaults for the INLINE and PREFIX clauses, see CREATE TABLE statement.

BLOB sharing

If a BLOB exceeds the inline size, and requires more than one database page for storage, the database server stores it so that it can be referenced by other rows in the same table, when possible. This is known as BLOB sharing. BLOB sharing is handled internally and is intended to reduce unnecessary duplication of BLOBs in the database.

BLOB sharing only occurs when you set values of one column to be equal to those of another column. For example, UPDATE t column1=column2;. In this example, if column2 contains BLOBs, instead of duplicating them in column1, pointers to the values in column2 are used instead.

When a BLOB is shared, the database server keeps track of how many other references there are to the BLOB. Once the database server determines that a BLOB is no longer referenced within a table, the BLOB is removed.

If a BLOB is shared between two uncompressed columns and one of those columns is then compressed, the BLOB will no longer be shared.

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