Since this is allowed at all times by Postgres this keyword has no effect. An SQL query which will provide the rows to be governed by the cursor. READ ONLY SQL92 keyword indicating that the cursor will be used in a readonly mode. Since this is the only cursor access mode available in Postgres this keyword has no effect. Since cursor updates are not currently supported in Postgres this keyword provokes an informational error message.
Cursors can return data either in text or in binary foramt.
Since data is stored natively in binary format, the system must do a conversion to produce the text format. In addition, text formats are often larger in size than the corresponding binary format. Once the information comes back in text form, the client application may have to convert it to a binary format to manipulate it anyway. So binary cursors will tend to be a little faster since they suffer less conversion overhead. User applications such as psql are not aware of binary cursors and expect data to come back in a text format.