Index Sequential Access Method

 

Indexed sequential access method (ISAM) files use a key file (ISI file) to determine which logical record in the physical text file is associated with the key data supplied. The ISI file acts as the control file for the processing of logical records within the physical text file. The key data is usually built from actual data within the logical record. However, the key data does not have to come from the logical record for ISAM processing.

Note the following:

  1. An ISAM file may permit or disallow duplicate keys within the ISI control file. If duplicate ISAM keys are not allowed, each logical record in the physical text file must have a unique ISAM key pointing to it. If duplicate ISAM keys are allowed, caution must be taken to insure the correct record is being processed since the same key information may be duplicated and reference a different logical record in the physical text file.

  2. An attempt to retrieve a logical record using a key that has not been previously written to the ISI file will fail and position the ISI file pointer to where the key information would have been placed had it been written.

  3. If fixed length records are used, the buffer size must be equal to or greater than the declared fixed record length. Any records written that are other than the declared fixed record length size (shorter or longer) will be padded or truncated as appropriate. In addition, deleted record space will automatically be re-used on ISI files with fixed record lengths.

  4. A physical text file may have more than one key file (ISAM and/or AAM) associated with it. However, caution should be exercised when processing physical text files with more than one key file associated with it to insure adjustments are made to each affected key file when any processing is performed.

  5. The ISI key information is placed into the ISI control file based on the standard ASCII collating sequence. The index method is a full B-tree structure supporting a maximum key size of 99 bytes and a maximum record length of 32,765 bytes.

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Sunaccess Reference Purpose of Sunaccess Associative Access Method