CHAR, VARCHAR, and BINARY columns can be compressed to save disk space. For example, you can compress a column in which large BLOB files such as BMPs and TIFFs are stored. Compression is achieved using the deflate compression algorithm. This is the same algorithm used by the COMPRESS function, and is also the same algorithm used for Windows ZIP files.
Compressed columns can reside inside of encrypted tables. In this case, data is first compressed, and then encrypted.
Do not use column compression on columns containing values under 130 bytes, or values that are already in a compressed format, such as JPG files. Attempting to compress columns that contain values that are already compressed may actually increase the amount of storage required for the column.
To compress columns, use the COMPRESS clause of the CREATE TABLE and ALTER TABLE statements.
You can determine the benefits you are getting by compressing columns using the sa_column_stats system procedure.
Discuter à propos de cette page dans DocCommentXchange.
|Copyright © 2012, iAnywhere Solutions, Inc. - SQL Anywhere 12.0.1|