Processing an ESDS in a SAS Job

Reading Records from an ESDS

You can use sequential or addressed direct access to read records from an ESDS within a SAS program. (See Access Types for ESDS Operations). If the ESDS has an alternate key index, you can also use keyed direct access. (See Keyed Direct Access by Alternate Keys.) The options that are specified in the INFILE statement determine the access type for an ESDS read operation. Either the VSAMLOAD or the VSAMUPDATE system option must be specified in order to read VSAM data sets.

Access Types for ESDS Operations
Operation	Read (INFILE/INPUT Statements)	Write (FILE/PUT Statements)
Read	Sequential	Does not apply
Read	Direct with RBA= option	Does not apply
Add*	Sequential with RBA= and SEQ options	Sequential: records are always added to the end of file
Add*	Direct with RBA= option	Sequential: records are always added to the end of file
Update	Sequential
Update	Direct with RBA= option	Direct: the last record read is the record updated
Load	Does not apply	Sequential: in entry order

* The INPUT statement is not required.

Reading an ESDS with Sequential Access

In an ESDS, sequential means in entry order. By default, the records that are in the data set are read from the beginning to the end of the data set. The following example shows the DATA step that you can use to read an ESDS sequentially:

/* Read data from an ESDS into a SAS data set   */

data one;
   infile myesds vsam; 
   input;
   ...more SAS statements...

If you specify the BACKWARD option, the data set is read backward, from the last record to the first record.

Reading an ESDS with Direct Access

Addressed Direct Access by RBA

When an ESDS is read with addressed direct access, records are retrieved directly by the address relative to the beginning of the data set (relative-byte address). For this type of access to be useful, you must know the RBAs of the records that you want to access. You might know the RBA if it has some relationship to the record contents or if you have obtained it (for example, from the _RBA_ automatic variable in a previous SAS DATA step).

To use addressed direct access, specify the RBA= option in the INFILE statement for the VSAM data set that is to be accessed by RBA. The RBA= option defines a variable whose value must be set to the RBA of the logical record or control interval to be retrieved by an INPUT statement. The address you specify must correspond to the beginning of a data record (logical record or control interval). Otherwise, the request is invalid and causes a VSAM logical error.

The following program illustrates addressed direct access to an ESDS:

data one;
   infile myesds vsam;
   input;
   rbanum=_rba_;
   keep rbanum;
run;

data two;
   set one;
   infile myesds vsam rba=rbanum;
   input;
   ...more SAS statements...

Keyed Direct Access by Alternate Keys

If there is an alternate key index for an ESDS, you can use keyed direct access by alternate keys to read an ESDS. An alternate index is created outside the SAS environment by using IBM Access Method Services. Then the ESDS records can be accessed directly by key, in a manner similar to a KSDS. (For an introduction to the alternate index concept, see Keyed Direct Access with an Alternate Index). For instructions on how to create an alternate index for an ESDS, see Using Alternate Indexes for VSAM Data Sets.

You can treat an ESDS accessed through an alternate index as if it were a KSDS, except that records cannot be erased and the record length cannot be changed.

The following table summarizes the type of access you can use for an ESDS with an alternate key index.

Operations on an ESDS with an Alternate Index
Operation	Access
Read	Sequential
Read	Direct by: alternate key RBA
Add	Sequential
Update	Sequential
Update	Direct by: alternate key, if required RBA
Load	Sequential

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