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Server-Side Cursor Advantage

Client-Side Cursor

Most SQL Servers gives you a client-side cursor only.

curs = db.SqlSelect( "SELECT * FROM T ..." )

Server sends found N records to a client computer as snapshoot, the records are not locked on the server side. You can only read the records of such client-side cursor.

When you want to update at least one field of this selection, you should

1) generate string of UPDATE command of kind

UPDATE TABLE T SET fld = NewValue WHERE KeyFld = current_id

To generate this string, usually few lines of programming code are required.

2) Execute this UPDATE command.

3) Get back changes, i.e.

  • destroy old cursor
  • build a new cursor using SELECT command with same conditions.

4) If you have some front-end to that cursor, you may need to take care to show again the current record.

Valentina DB (local and server) also have client-side cursors. But it also offers server-side cursor.

Server-Side cursor of Valentina DB

In Valentina API you can ask for server-side cursor as a result.

curs = db.SqlSelect( "SELECT * FROM T ...", kServerSideCursor, kReadWrite )

In this case Valentina DB Server does not send all found records to a client computer. It sends one or few. So the first possible case for use of this cursor is - huge selections.

Besides, Valentina Server keeps record locks on selected rows of cursor. This allows to modify records!

As a result, it can be much easier to build front-end around server-side cursor. To modify field you should just

curs.Field(Name).value = NewVaue
curs.UpdateRecord()

DONE.

Please note, that in this case THERE IS NO ANY SQL strings built behind this!

VCursor.UpdateRecord() is true protocol-based client-server command of Valentina API.

Advantage

Valentina's solution removes overhead of these operations:

  • client: build UPDATE string generation using concatenation
  • client: sending UPDATE string by protocol
  • server: parsing of that string on server side
  • server: send back result of UPDATE command
  • client: sending of SELECT string
  • server: parsing of SELECT string
  • server: Executing of SELECT
  • server: sending back N records by protocol
  • client: extract records from protocol and build cursor with records in RAM structures.