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  Cursors and Data management

Cursors:

A cursor is "opened” on a specific set as well as thereafter points exclusively to records in that set. Subsequent to a cursor is opened it may be copied, moved or closed. Whereas a cursor is opened it may be utilized to manipulate the record it addresses.

Records are addressed beside cursors. Cursors provide the functions of

  • Pointing at a record.
  • Enumerating all records in a set.
  • Translating among the stored record format and the format visible to the cursor user. An easy instance of this might be a cursor that hides some fields of a record. This element will be discussed with the notion of view.

A cursor is an transient object that is created from a descriptor when a transaction is initiated or during transaction execution by an explicit OPEN_CURSOR command. As well one may COPY_CURSOR a cursor to make another instance of the cursor with independent positioning. A cursor is opened on a exact set (which thereby defines the enumeration order (next) of the cursor.) A cursor is shattered by the CLOSE_CURSOR command.

Operations on cursors:

Operators on cursors comprise:

FETCH (<cursor> [, <position>]) [HOLD] RETURNS(<record>)

This retrieves the record pointed at by the named cursor. The record is enthused to the specified target. If the position is precise the cursor is first positioned. If HOLD is specific the record is locked for update (exclusive) or else the record is locked in share mode.

INSERT (<cursor>[, <position>], <record>)

Inserts the accurate record into the set specified by cursor. If the set is key sequenced or else entry sequenced then the cursor is moved to the correct position before the record is inserted otherwise the record is inserted at (after) the current position of the cursor in the set. If the record type automatically emerges in other sets, it as well inserted in them.

UPDATE (<cursor> [, <position>],<new_record>)

If position is précised the cursor is first positioned. The new record is subsequently inserted in the set at the cursor position replacing the record pointed at by the cursor. If the set is sequenced in the updated fields this may cause the record and cursor to move in the set.

DELETE (<cursor> [, <position>])

Deletes the record pointed at by the cursor subsequent to optionally repositioning the cursor.

MOVE_CURSOR (<cursor>, <position>) HOLD

Repositions the cursor in the set.

Cursor positioning:

A cursor is opened to navigate a particular set and positioning expressions have the syntax

--+-------------<RID>-------------------------------------------+-------------+-;
  +-------------FIRST--------------------------------------------+ |
 +--------------N-TH---------------------------------------------+    +-CHILD-----+
 +--------------LAST --------------------------------------------+---+---------+
 +---NEXT-------+                                                              +   +-PARENT--+
 +--PREVIOUS--+-<SELECTION EXPRESSION>------+   +-GROUP---+
 +------------------+

Where RID, FIRST, N-th, as well as LAST specify specific record occurrences while the other options specify the address relative to the current cursor position. It is as well possible to set a cursor from another cursor.

The selection expression may be whichever Boolean expression valid for all record types in the set. The selection expression comprises the relational operators: =,!=, >, <, <=, >= and for character strings a ‘matches-prefix’ operator sometimes called generic key. If next or else previous is specified, the set should be searched sequentially because the current position is relevant. Or else the search can employ hashing or indices to locate the record. The selection expression search perhaps performed via an index which maps field values into RIDs.

Illustration of commands are:

FETCH (CURSOR1, NEXT NAME='SMITH') HOLD RETURNS (POP);
DELETE (CURSOR1, NEXT NAME='JOE' CHILD);
INSERT (CURSOR1, , NEWCHILD);

For partitioned sets one may perhaps point the cursor at a specific partition by qualifying these operators by adding the modifier GROUP. A cursor on a parent-child or else partitioned set points to both a parent record and a child record or group and child within group. Cursors on such sets contains two components- the parent or group cursor as well as the child cursor. Poignant the parent curser positions the child cursor to the first record in the group or under the parent. For parent-child sets one succeeds the position operator with the modifier NEXT_PARENT in order to locate the first child of the next parent or with the modifier WITHIN PARENT if the search is to be restricted to children of the current parent or group. Or else positional operators operate on children of the current parent.

There are rather unclear issues associated with cursor positioning.

The subsequent is a good set of rules:

a) A cursor is able to have the subsequent positions

  • Null.
  • Before the first record.
  • At a record.
  • Between two records.
  • After the last record.

b) If the cursor points at a null set subsequently it is null. If the cursor points to a non-null set afterwards it is always non-null.

c) Initially the cursor is prior to the first record unless the OPEN_CURSOR specifies a position.

d) An INSERT operation abscond the cursor pointing at the new record.

e) A DELETE operation leaves the cursor among the two adjacent records or at the top if there is no previous record or at the bottom if there is a previous however no successor record.

f) A UPDATE operation absconds the cursor pointing at the updated record.

g) If an operation not succeed the cursor isn’t altered.

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