One reason an MFS may wish to trap the READNEXT call is in order to fulfill READNEXT calls without recourse to the BFS. In OpenInsight, both QUICKDEX.MFS and SI.MFS trap the READNEXT call, returning record keys without passing the call to the BFS.
QUICKDEX.MFS provides a good model for using an MFS to trap READNEXT requests. If the filing system SELECT call has returned a select mode of 1 (latent file select), QUICKDEX.MFS traps the READNEXT call. The MFS reads the %RECORDS% record from the current file, and returns it as the next block of keys. If another READNEXT call comes down to the MFS, QUICKDEX.MFS simply returns a false, since all keys were returned in the first call.
Indexed-Based SELECT
If the sort criteria passed in an extended BASIC+ SELECT statement represent indexed fields, there is no need for the SELECT processor to execute an entire SELECT ... READNEXT ... READ loop in order to build a resolved list of keys. Instead, the list can be constructed directly from the Btree index.
The result of this condition is a latent index-based select. This is similar to a latent file-based select condition as established by a simple BASIC+ SELECT. However, it differs in that a latent index-based select can return keys in sorted order.
A latent index-based select condition is not a BFS-level function. If an indexed field is passed as a sort criterion in a filing system SELECT call, SI.MFS traps the call. Other MFSs that follow SI.MFS, and the actual BFS, never receive the SELECT call. SI.MFS returns a select mode of 2 in the RECORD argument (causing @LIST.ACTIVE to be set to 2), indicating an index-based select condition.
SI.MFS will then trap subsequent filing system READNEXT calls, providing blocks of keys out of the Btree index. Again, filing systems that follow SI.MFS will never receive this call, since it is trapped and fulfilled by SI.MFS.
CLEARSELECT MFS Processing
A special call is provided to enable a filing system to clear select lists. The major function of a CLEARSELECT is to clear the system variables (such as the system list variable) used to indicate an active select list.
The call is passed through to the filing system, however, in case the filing system needs to take action when a select list condition is cleared. For example, Linear Hash (RTP57) uses a CLEARSELECT call as an indication that it can reset its sizelock parameter by decrementing it.
