When creating an ISAM file, you can specify that you want your file to compress data. A repeated string of characters can be compressed to a few bytes. Compression can save from 10 percent to 80 percent of your disk space requirements for the data portion of an ISAM file (.is1), with no program changes. Records containing text fields are ideal candidates for compression. You can compress fixed-length or variable-length files but not multiple fixed-length files.
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If you require that your RFAs remain the same on WRITE operations, you must use static RFAs. (Static RFAs are required with Revision 6 ISAM files.) With data compression, your compressed data record size may change, causing the RFA to change. Do not use data compression if you expect a record’s RFA to remain the same after a WRITE, unless you build the file with static RFAs as well. (In other words, your response to the bldism prompt “Enter name of the ISAM file to create:” would be filename,compress,static_rfa.) This is especially true if you use manual locking. If you use xfODBC and queries that have joins that do not use a default index (i.e., joins for which xfODBC must create temporary indexes), you must use static RFAs on your files. |
