Internal Algorithms

SELECT

When a SELECT is executed, the editor pulls in data from the table with BULK FETCH statements, 12K bytes at a time. If more than one BULK FETCH is needed, it appends the data from the previous FETCH to the scroll file. After the last FETCH in a sequence of FETCH statements, the data is appended to the scroll file, and the first 12K of data is read back into the data buffer. If all the data fits in the buffer, the scroll file is not used at all. Before control returns to the user, the transaction is ended.

The window pointer is then set to the first row in the buffer, and the first set of rows is displayed. Cursor positioning and page scrolling, since they are handled completely by the editor without accessing the database, are very fast.

DELETE

When a DELETE is executed, the tuple is fetched from the table based on its old primary key value. If the tuple is not found, then the following message is returned:

   Tuple Does Not Exist

The row is marked as deleted in the memory buffer, and the table is redisplayed starting from the window pointer. All rows marked as deleted are skipped by the display manager.

If the tuple to be deleted is still found in the table, it is compared against the old value of the tuple. If they are identical, the tuple is deleted from the table, the row is marked as deleted in the memory buffer, and the table is redisplayed starting from the window pointer.

If the tuple in the database has changed, then the following message is returned:

   Tuple Has Changed

The tuple's value in the buffer is updated, and the table is redisplayed starting from the window pointer.

In all three cases, the transaction is ended before control returns to the user. Locks are obtained and released before additional terminal I/O is requested.

UPDATE

When an UPDATE is executed, the tuple is fetched from the table based on its old primary key value. If the tuple is not found, then the following message is returned:

   Tuple Does Not Exist

The row is marked as deleted in the memory buffer, and the table is redisplayed starting from the window pointer.

If the tuple is found, it is compared against the old value of the tuple. If they are identical, the tuple is updated, the tuple's value in the buffer is updated, and the table is redisplayed starting from the window buffer.

If the tuple in the database has changed, then the following message is returned:

   Tuple Has Changed

The tuple's value in the buffer is updated, and the table is redisplayed starting from the window pointer.

In all three cases, the transaction is ended before control returns to the user.

	CAUTION: A dirty bit is kept for the whole data buffer. The bit is set when the contents of the buffer is modified (that is, becomes "dirty"). If set, the current contents have to be written to the scroll file before a new 12K block is read from it.