Simple-Large-Object Storage and the Descriptor

Data rows that include TEXT or BYTE data do not include the data in the row itself. Instead, the data row contains a 56-byte descriptor with a forward pointer (rowid) to the location where the first segment of data is stored.

The descriptor can point to one of the following items:

• A page (if the data is stored in a dbspace)
• A blobpage (if the data is stored in a blobspace)
• An optical platter (if you are using the Optical Subsystem)

Creation of Simple Large Objects

When a row that contains TEXT or BYTE data is to be inserted, the simple large objects are created first. After the simple large objects are written to disk (or optical medium), the row is updated with the descriptor and inserted.

Deletion or Insertion of Simple Large Objects

The database server cannot modify simple large objects. It can only insert or delete them. Deleting a simple large object means that the database server frees the space consumed by the deleted object for reuse.

When TEXT or BYTE data is updated, a new simple large object is created, and the data row is updated with the new blob descriptor. The old image of the row contains the descriptor that points to the obsolete value for the simple large object. The space consumed by the obsolete simple large object is freed for reuse after the update is committed. Simple large objects are automatically deleted if the rows that contain their blob descriptors are deleted. (Blobpages that stored a deleted simple large object are not available for reuse until the logical log that contains the original INSERT record for the deleted simple large object is backed up. For more information, see backing up logical-log files to free blobpages in the chapter on what is the logical log in the IBM Informix Administrator's Guide.)

Size Limits for Simple Large Objects

The largest simple large object that the blob descriptor can accommodate is (2³¹ - 1), or about 2 gigabytes.