DYNSLT (Dynamic Select) keyword—logical files only

You can use this file-level keyword to indicate that the selection and omission tests specified in the file (using select/omit specifications) are done at processing time.

This keyword specifies dynamic select/omit rather than access path select/omit. This keyword has no parameters.

When your program does input operations to a logical file with the DYNSLT keyword specified, all the records in the associated physical file are tested by the system to see if they satisfy the select/omit values. Only those records that satisfy the values are supplied to your program. The testing of each record can result in slower I/O performance, but can be more efficient than maintaining an access path for the file. This is particularly likely for files read only occasionally, especially when the physical files they are based on are updated frequently. Using dynamic select/omit is probably also more efficient for files with a high percentage of selected records.

In keyed sequence access files, an access path is created at file creation time and is maintained for the file according to the MAINT parameter on the Create Logical File (CRTLF) or Change Logical File (CHGLF) command. The DYNSLT keyword does not affect the maintenance of access paths for keyed sequence access files.

For all single-format logical files with a DYNSLT keyword, you do not need to specify key fields in order to specify select/omit fields. However, for all multiple-format logical files with a DYNSLT keyword, you do need to specify at least one key field. You can specify *NONE for this key field.

You must use the DYNSLT keyword when you want to select or omit fields and any of the following situations are true:

The logical file has arrival sequence (no key fields are specified). See example 1 in this topic.
The logical file is a join logical file with the JDFTVAL keyword specified.
The logical file is a join logical file, select/omit fields come from more than one of the physical files the logical file is based on, and one of the following conditions is true:
- The select/omit fields are on the same select or omit statement. See example 3 in this topic.
- The select/omit fields are on a mixture of select and omit statements. See example 4 in this topic.
- The select/omit fields are on select statements that are grouped together by OR.
- The select/omit fields are on omit statements that are grouped together by AND.

You cannot specify the DYNSLT keyword with the REFACCPTH keyword.

For a join logical file, the select/omit fields can occur in any of the physical files specified on the JFILE keyword. Use the JREF keyword in join logical files to qualify the origin of the field and resolve any ambiguities.

Examples

The following examples show how to specify the DYNSLT keyword.

Example 1

The following example shows how to specify dynamic select with arrival sequence.

|...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8
00010A                                      DYNSLT
00020A          R RECORD1                   PFILE(PF1)
00030A            FLD1
00040A            FLD2
00050A          S FLD1                      COMP(GT 2)

The DYNSLT keyword is required because there are no key fields.

The logical file supplies records to your program in arrival sequence. Assume that physical file PF1 has the following records:

FLD1: FLD2
1: aaaa
2: dddd
3: jjjj
4: bbbb

As your program does input operations, the system tests the first two records according to the select/omit values, but does not supply them to your program. Your program only sees the last two records:

FLD1: FLD2
3: jjjj
4: bbbb

Example 2

The following example shows how to specify dynamic select with keyed sequence access path.

|...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8
00010A                                      DYNSLT
00020A          R RECORD1                   PFILE(PF1)
00030A            FLD1
00040A            FLD2
00050A          K FLD1
00060A          S FLD2                      COMP(GT 'bbbb')
     A

In this example, the DYNSLT keyword is not required. The logical file supplies records to your program in keyed sequence. Assume that physical file PF1 has the following records:

FLD1: FLD2
1: aaaa
2: dddd
3: jjjj
4: bbbb

When your program requests a record, the system tests the value of FLD2 for that record according to the select/omit values. Your program only sees the following records:

FLD1: FLD2
2: dddd
3: jjjj

Example 3

The following example shows how to specify a join logical file with select/omit comparing fields from two physical files.

|...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8
00010A                                      DYNSLT
00020A          R RECORD1                   JFILE(PF1 PF2)
00030A          J                           JFLD(FLD1 FLD3)
00040A            FLD1                      JREF(PF1)
00050A            FLD2                      JREF(PF1)
00060A            FLD3                      JREF(PF2)
00070A            FLD4                      JREF(PF2)
00080A          S FLD1                      COMP(GT FLD4)
     A

FLD1 and FLD2 come from the primary file (PF1), and FLD3 and FLD4 come from the secondary file (PF2). The select specification compares FLD1 from the primary file with FLD4 from the secondary file. Therefore, the DYNSLT keyword is required.

Example 4

The following example shows how to specify a join logical file with select and omit using fields from more than one physical file.

|...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8
00010A                                      DYNSLT
00020A          R JREC                      JFILE(PF1 PF2)
00030A          J                           JOIN(PF1 PF2)
00040A                                      JFLD(FLD1 FLD2)
00050A            FLD1                      JREF(PF1)
00060A            FLD2                      JREF(PF1)
00070A            FLD3                      JREF(PF2)
00080A          K FLD1
00090A          S FLD1                      COMP(GT 0)
00100A          O FLD3                      COMP(GT 4)
     A

FLD1 and FLD3 come from different physical files and are specified in a mixture of select and omit statements. Therefore, the DYNSLT keyword is required.