Advanced User Topics for the SAS/ACCESS Interface View Engine for IMS

Understanding the IMS Interface

This section describes concepts that are exclusive to the SAS/ACCESS interface to the IMS engine. You must understand these concepts in order to successfully use the interface. This section describes the following concepts:

flattened file concept
missing values
special fields
BY variables

Understanding the Flattened File Concept

When the IMS engine creates SAS observations from a hierarchical database, it must flatten out the data. The flattened file concept means that SAS flattens the hierarchical levels and treats one path of data, including the root segment, parent segments, and child segments, as one SAS observation. If the root segment or any parent segment has children, the parent segment is repeated for each child segment's data. Therefore, each observation contains all the parent segments above the child segment.

For example, if you access the data in the database shown in Flattened File Concept, the IMS engine will return data from the segments in the following table as SAS observations. Therefore, the view descriptor would have to define four segment types.

Flattened File Segments
Observation	Segments returned
1	1 2 3 4
2	1 2 3 5
3	1 2 3 6
4	1 2 3 7
5	1 8 9 .
6	1 8 10 .

Flattened File Concept

[Flattened File Concept]

If you use the data from these observations in a SAS procedure, it appears that the data in segment 1 occur six times rather than only once. This can result in misleading statistics when you use such procedures as the MEANS procedure that involves any segment except the child segment in a database with more than two hierarchical levels. It can also be a problem in second-level data because root data repeats. To avoid misleading statistics that can result from flattened files, create view descriptors that describe data in only one hierarchical level. Or perform statistical operations using data from only the lowest level that is accessed by the view descriptor.

Using the *U Command Code

The IMS engine generates navigational SSAs to traverse and flatten the database hierarchy. Because sequential calls perform this task, the database's current position is an important issue. (See Database Position for more information.)

Using a *U command code ensures the current database position on the proper parent segment as a DL/I call moves down the hierarchy to the next target segment (the segment named in the last SSA). *U on the immediate parent of the target means that even if the parent is unqualified, the position indicator remains there and does not move to a child (target segment) that belongs to a different parent occurrence.

For example, when DL/I processes a Get or ISRT call, it establishes a position on the segment occurrence that satisfies the call at each level in the path of the segment (target) that you are retrieving or inserting. A *U command code on an SSA in a Get or ISRT call tells DL/I not to move from the established database position at the level of the SSA when trying to satisfy the call. You use an ISRT call and I/O or TP PCBs to insert messages to the IMS/ESA control region message queues when a SAS program is executing in a BMP region. See ISRT Calls to Message Queues.

Handling Missing Values

This section describes how the SAS/ACCESS interface to IMS handles missing data values. It also describes how the DB Content field affects how data is displayed and stored in the database.

If you create a view descriptor to add an IMS-DL/I database segment and fields in that segment are not defined, the IMS engine writes low values to the database fields that are not included in the descriptor. The engine does so because it does not know that the fields exist.

If there are missing values in a SAS data set that you use to add or update an IMS database, the IMS engine writes zeros to the database for numeric fields and blank spaces to the database for character fields unless you specify a special format (B, L, or H) for the DBCONTENT= argument of the ITEM= statement. DBCONTENT= affects how the engine updates the fields. (See ITEM= Statement for more information about special formats.)

Conversely, if a field is defined with a DBCONTENT= value and the database retrieves that value (blanks, low values, or high values) in the field, then the IMS engine passes missing values to SAS. In addition, if a view descriptor describes more than one level in a database, and not all the levels exist for one database record, the IMS engine fills the missing segment occurrence with missing values in the SAS observation.

Using BY Variables

If you specify an IMS view descriptor as input to a SAS procedure that uses a BY variable, you must either

create a SAS data file from the IMS data (that is, extract the data) and sort the data using that variable. You then specify the newly created data file in your procedure.
reference a SAS variable associated with a database index in the BY statement; that is, the BY variable must be defined as the index key.

Handling Special Fields

Handling Fields That Occur Multiple Times

An item or a group in an IMS database segment can occur more than one time. For example, in the example database ACCTDBD, the two phone number fields, home phone and office phone, could be defined in your access descriptor as one field that occurs two times. To do this, specify OCCURS=2 in the ITEM= statement for the phone number field when you create the access descriptor. When you save the access descriptor, the descriptor is expanded to show fields for two phone numbers. When the IMS engine reads the database, it retrieves two phone numbers for each customer.

Fields that occur multiple times in the database can be nested only three levels deep, which creates a three-dimensional table. The following example shows the definition of a record with fields that occur multiple times, nested three levels deep:

01 Automatic Teller Record
   02 ATM Information
     03 ATM Location (occurs 20 times)

       04 Location
       04 ATM Transaction Information (occurs 7 times)

          05 Account Type
          05 Transaction Time
          05 Transaction (occurs 2 times)

             06 Transaction Type
             06 Transaction Amount

After you have saved an access descriptor, you cannot change the number in the OCCURS= argument. Instead, you have to delete an item and re-enter it with the correct number in OCCURS=.

Handling Redefined Fields

Redefined fields are fields that have been defined with more than one data type. For example, some records in a database might have character values stored in a certain field, and other records in the same database might have numeric values stored in that same field. You could handle this by defining the field as $11. in one access descriptor and 11. in another access descriptor based on the same database. When you create view descriptors for the database, use a WHERE statement to retrieve only the appropriate values for the field. This can often be done by specifying a particular record type or other code in the WHERE statement.

Handling Segments of Varying Length

If you work with a segment that contains a field that varies in length, specify the maximum length of the varying field for SEGLNG= when you define the segment in the access descriptor. When IMS retrieves the entire segment, it fills in the varying portion with missing values if it did not retrieve any data for that portion of the segment.

Handling GROUP Keys in Descriptor Files

To support a definition of a GROUP field as a key and to be able to have access to the GROUP items, you need to define a dummy field for this key.

In IMS, GROUPs enable the same portion of data in a buffer to be assigned different logical names. For example, a field that begins at offset 1 for a length of 15 can be named FIELD1. Other fields can be defined within FIELD1, such as in FIELD2, FIELD3, and FIELD4 that begin at offsets 1, 6, and 11, respectively (where each has a length of 5).

Because no SAS variable name can be specified in the GROUP= statement, no single reference can be made to the group in the WHERE criteria. Therefore, even if a valid SEARCH or SEQ name exists for the GROUP in the DBD, the IMS engine cannot qualify calls that are based on the group itself.

A simple solution is to define the entire group as an item and to assign the SAS variable name and SEARCH name appropriately. Then you can specify a WHERE statement in your view descriptor or application and the IMS engine will build qualified SSAs. A problem remains if the application wants access to the components of the GROUP. In this case, you must reference the view descriptor in a DATA step to SUBSTR out the parts and store them in separate SAS variables.

Using Dummy Fields for GROUP Keys

You can define a dummy field in the segment for a GROUP key in order to permit a WHERE clause reference for qualified SSAs and to access the composite fields. The GROUP statement defines the group but you can take it a step further. You add a dummy field to the end of the segment definition as an ITEM with a length that is equal to the entire GROUP and a SEARCH= value equal to the DBD SEARCH or SEQ field name from the DBD (the GROUP SEARCH= also has this value). The SEGLNG value is increased for this field.

By using a dummy field for the GROUP, you can specify in your view descriptor a WHERE clause as follows:

WHERE sas-dummy-name EQ value

In this case, the IMS engine locates the dummy field in the view descriptor through the SAS variable name in the WHERE clause. It uses its SEARCH= value to qualify the SSA. When the data comes back to the buffer, the true data is in the GROUP portion of the segment definition and its component values are stored in the SAS variables that are associated with the items that are defined for the GROUP.

Also, by marking the GROUP itself as the key (with the KEY= argument), navigational SSAs that are generated by the IMS engine for sequential GN calls will refer to the correct buffer location for the data. The navigational SSAs will use the correct SEARCH= value in the SSA.

CAUTION:: You must never refer to the dummy field as the key (with KEY=) because doing so would force the IMS engine to use the dummy buffer location to qualify navigational SSAs for GN calls. This would cause problems.

Below is an example of an access descriptor and a view descriptor based on the ACCTDBD. The GROUP key is on home phone, which has a dummy field (GROUP STUFF) defined for it.

proc access dbms=ims;
  create work.account.access;
    dbd=acctdbd dbtype=hdam;
    record='customer_record' sg=customer sl=225;
      item=soc_sec_number   lv=2 dbf=$11. key=u
                            se=ssnumber;
      item=customer_name    lv=2   dbf=$40. 
                            se=custname;
      item=addr_line_1      lv=2   dbf=$30.  
                            se=custadd1;
      item=addr_line_2      lv=2   dbf=$30.  
                            se=custadd2;
      item=city             lv=2   dbf=$28.  
                            se=custcity;
      item=state            lv=2   dbf=$2.   
                            se=custstat;
      item=country          lv=2   dbf=$20.  
                            se=custland;
      item=zip_code         lv=2   dbf=$10.  
                            se=custzip;
     group=home_phone       lv=2           
                            se=custhphn;
      item='area code'      lv=3   dbf=$3.   
      item=filler1          lv=3   dbf=$1.   
      item=phone_number     lv=3   dbf=$8.   
      item=office_phone     lv=2   dbf=$12.  
                            se=custophn;
      item='group stuff'    lv=2   dbf=$12.  
                            se=custhphn;
  list all;

  create work.phone.view psbname=acctsam pcbindex=2;
    select soc_sec_number customer_name 'area code' 
       'phone number' 'group stuff';
  list view;
run;

proc print data=work.phone;
  var soc_sec_number customer_name 'area code' 
      'phone number';
  where 'group stuff' = '803-657-1346' or 
        'group stuff' = '803-657-1687';
run;

The following output shows the results.

Results of a Dummy Field for a GROUP Key

                                  The SAS System

OBS   soc_sec_number           customer_name          'area code'  'phone number'

  1    436-42-6394             BOOKER, APRIL M.        803          657-1346
  2    178-42-6534             PATTILLO, RODRIGUES     803          657-1346
  3    434-62-1234             SUMMERS, MARY T.        803          657-1687

Using Filler Notation in ITEM=

It is important that access descriptor segment definitions not omit ITEM and GROUP references for fields that are embedded in the segment. Database segments might contain fields (contiguous or discontiguous) that applications might not need to access. In these cases, it is correct not to define them in SAS/ACCESS view descriptors. For performance reasons, it is recommended that applications not define them so that the IMS engine does not invoke conversion routines to convert data that will not be used.

Sites commonly refer to undesired portions of the data buffer by using the FILLER notation in the ITEM= statement, and by defining the DBC (DB Content) as $CHAR. If the undesired portion of the segment lays beyond all the desired segment fields, applications do not have to define these portions of the segment. However, you must make sure that the SEGLNG value for the segment is equal to the length of the entire segment and not just to the portion of the segment that they are interested in defining.

When the undesired fields are embedded between desired fields, you must use the FILLER notation or something similar (FILLER is a reserved word in COBOL but not in SAS). SAS uses relative offsets to locate defined fields in the buffer when converting data from the IMS buffer to the SAS program data vector (PDV). By using the field lengths from the DBC, SAS determines the offset and length in the IMS buffer for the current field as needed to map to the PDV. If a field or series of fields is undesired, information must be supplied about placement and length so that SAS can move correctly to the next valid field to be mapped.

FILLER fields can be coded as DBC of $CHAR, which requires no conversion if selected for a view descriptor. In most cases FILLER fields are not selected. By preserving the relative offsets of fields within the buffer using FILLER definitions, the IMS engine can correctly map data that is requested by the application or view descriptor to the PDV.

Below is an example of a root segment for the ACCOUNT database with all of the fields defined from the DBD.

record='customer_record'   segment=customer 
                           seglng=225;
  item=soc_sec_number      lv=2  dbf=$11.  
                           search=ssnumber key=y;
  item=customer_name       lv=2   dbf=$40.  
                           search=custname;
  item='address info'      lv=2;
  item=addr_line_1         lv=3   dbf=$30.;
  item=addr_line_2         lv=3   dbf=$30.;
  item=city                lv=3   dbf=$28.;
  item=state               lv=3   dbf=$2. ;
  item=country             lv=3   dbf=$20.;
  item=zip_code            lv=3   dbf=$10.;
  item=home_phone          lv=2   dbf=$12.;
  item=office_phone      lv=2   dbf=$12.;

Assuming that none of your view descriptors would ever require phone information, you could code the following:

record='customer_record'   segment=customer 
                           seglng=225;
  item=soc_sec_number      lv=2  dbf=$11.  
                           search=ssnumber key=y;
  item=customer_name       lv=2   dbf=$40.  
                           search=custname;
  item='address info'      lv=2;
  item=addr_line_1         lv=3   dbf=$30.;
  item=addr_line_2         lv=3   dbf=$30.;
  item=city                lv=3   dbf=$28.;
  item=state               lv=3   dbf=$2. ;
  item=country             lv=3   dbf=$20.;
  item=zip_code            lv=3   dbf=$10.;

Note that the SEGLNG= value does not change even though two fields at the end are dropped.

By comparison, assume that the application needs everything except the address information:

record='customer_record'   segment=customer 
                           seglng=225;
  item=soc_sec_number      lv=2   dbf=$11
                        .  search=ssnumber key=y;
  item=customer_name       lv=2   dbf=$40.  
                           search=custname;
  item='filler'            lv=2   dbf=$char60.;
  item=city                lv=3   dbf=$28.;
  item=state               lv=3   dbf=$2. ;
  item=country             lv=3   dbf=$20.;
  item=zip_code            lv=3   dbf=$10.;
  item=home_phone          lv=2   dbf=$12.;
  item=office_phone        lv=2   dbf=$12.;

Here, the FILLER preserves 60 bytes so that view descriptors that reference fields past the filler can get data mapped correctly from the IMS buffer to the PDV variables based on the relative offset information. Once again, SEGLNG= does not change.

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