Interpreting the Scan Descriptor

The database server constructs a scan descriptor to pass the contents of the SELECT clause to the access method. The scan descriptor specifies which columns the query projects and provides information about the locks and isolation levels that apply to the data that the query specifies.

For efficiency, the access method can format only the data that SELECT clause projects and places NULL values in the remaining columns. To determine which columns to project, call the mi_scan_projs() function.

As one of its primary functions, the scan descriptor stores a pointer to another opaque structure, the qualification descriptor that contains WHERE-clause information. To access the qualification descriptor, use the pointer that the mi_scan_quals() function returns. A NULL-valued pointer indicates that the database server did not construct a qualification descriptor.

For more information about the information that scan descriptor provides, refer to Descriptor Function Reference and the scan-descriptor accessor functions that begin with mi_scan_forupdate().