Storing several tables in a single Adabas file

[Patricio]

In some cases, our legacy data has more than one table stored in a single physical file. What problems might I encounter?

Cheers,
Patricio

[Andre]

Storing several tables in the same physical Adabas file was a common practice, especially when file numbers were limited to 255 files in a database. When accessing these tables using AIS, users usually prefer to map the single physical file to several logical tables. Normally Predict views or DDM definitions exist with this logical mapping. Attunity Connect uses these definitions and only sees the columns and descriptors relevant to the logical table being accessed. Since it is common practice to make all the fields in such a file NULL suppressed, using a descriptor is also fine because only records or that logical table are part of the descriptor's inverted list. The only problem in providing this logical mapping is when the Attunity Query Processor decides to scan such a logical table. Using L2 will return lots of ’empty’ rows corresponding to the records of the other logical tables in the file.

In order to solve this problem, it is possible to specify that a scan strategy use L3 with a particular descriptor instead of L2. This option is currently supported only using the ADD-Adabas driver. To set it, you just need to specify the descriptor name to be used for scanning the logical table in the table's dbCommand, next to the file number. For example, <dbCommand>17;AA</dbCommand> will cause L3 on the AA descriptor to be used for scanning the table. Care must be taken to make sure that the descriptor selected is such that all records belonging to the logical table will be accessible using the descriptor's inverted list.

Hope this helps,
Andre