Scan Descriptor

The scan descriptor, or MI_AM_SCAN_DESC structure, contains the specifications of an SQL query, including the following items:

• The columns to project
• A pointer to selection criteria from the WHERE clause
• Isolation and locking information
• A pointer to where the access method can store scanned data

The database server passes the scan descriptor to the access-method scanning purpose functions: am_beginscan, am_endscan, am_rescan, and am_getnext.

The following functions extract information from the scan descriptor.

Accessor Function

Return Value

mi_scan_forupdate()

MI_TRUE if a SELECT statement includes a FOR UPDATE clause

mi_scan_isolevel()

The isolation level for the table

mi_scan_locktype()

The lock type for the scan

mi_scan_newquals()

MI_TRUE if the qualification descriptor changes after the first scan for a join or subquery

mi_scan_nprojs()

The number of columns in the projected row that the access method returns to the query

mi_scan_projs()

A pointer to an array that identifies which columns from the row descriptor make up the projected row that the query returns

mi_scan_quals()

A pointer to the qualification descriptor or a NULL-valued pointer if the database server does not create a qualification descriptor

mi_scan_table()

A pointer to the table descriptor for the table that the access method scans

mi_scan_userdata()

A pointer to the user-data area of memory

The following accessor function sets data in the qualification descriptor.

Accessor Function

Value Set

mi_scan_setuserdata()

The pointer to user data that a subsequent function will need