Efficient File Access Techniques

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Previously, we saw how a file can be read using the FileAdapter and AdgDataSet objects. FileAdapter’s ReadSequential method is the best way to read each record of a file in succession. For large files, using this method to find a particular record of interest can be inefficient. The ReadRandomRRN, ReadRandomKey, ReadRange, and ReadSequentialEqual methods are provided for this reason.

ReadRandomRRN reads a record specified by relative record number (RRN). ReadRandomRRN may not be implemented by all DG database providers, but it allows “random” access to files that may be otherwise non-indexed. On the other hand, ReadRandomKey , ReadRange , and ReadSequentialEqual are methods for reading files that are indexed by key data. To use these, one or more data keys must be initialized and passed to the method.

In DG, key and record data are modeled as DataRow objects. DataColumn objects in the DataColumnCollection of the DataTable corresponding to the record format or key contain the basic type information for each field. The ActiveRow property of AdgDataSet is a reference to the DataRow object representing the data in the record accessed.

Likewise, a DataRow object can represent a key. Generally, to obtain a DataRow object for a particular record format’s key you would use the NewKeyTable method of AdgDataSet. Use this method on the AdgDataSet object returned by the OpenNewAdgDataSet method of FileAdapter . NewKeyTable returns an AdgKeyTable object reference containing the DataRow object for the key.

The Row property of AdgKeyTable references this DataRow . As with the PrepareRow methods of AdgDataSet , the DataRow referenced by AdgKeyTable will be initialized with “null values”. Before using the new key in a ReadRandomKey method call, the Value properties should be set to reflect the key being sought, as in the following example.

  AdgConnection cx;
  FileAdapter DbFile;
  AdgDataSet Ds;
  AdgKeyTable KeyTbl;
  Cx = new AdgConnection("ASNA Local DB");
  Cx.Open();
  DbFile = new FileAdapter(Cx,"Asna_example_files/custmast","*first");
  DbFile.AccessMode = AccessMode.RWCD;
  DbFile.OpenNewAdgDataSet( ref Ds );
  KeyTbl = Ds.NewKeyTable(ds.GetFormatName(0));
  KeyTbl.Row["CMCustNo"] = 500;
  DbFile.ReadRandomKey( Ds, ReadRandomMode.GTEQ, LockRequest.Default, KeyTbl );
  Console.WriteLine( "CMCustNo = " + Ds.ActiveRow["CMCustNo"] ) ;

In the above example, we access a file using the ReadRandomKey method. This particular file has one key field named “CMCustNo”. The NewKeyTable method is called on the AdgDataSet returned from OpenNewAdgDataSet. NewKeyTable is passed the format name of the only format of the file. Note that we use the GetFormatName method of AdgDataSet to avoid hard-coding the format name. The returned AdgKeyTable object contains one DataRow object consisting of only one database field value for the CMCustNo field. This key field is initialized with the integer value 500 in this example. Next, ReadRandomKey is called passing the key value in the AdgKeyTable object. Also note that the ReadRandomMode.GTEQ is passed to ReadRandomKey . This specifies that we would like to read the next record containing the given key, or otherwise the next record containing a key greater than the given key.

Efficient File Access Techniques

See Also