Verbs and Built-in Functions › &NDBSCAN › Scan Processing

Scan Processing

A scan is processed in four phases:

1. The scan-expression is parsed, and an action table built.
2. The action table is processed, using keys wherever possible, to obtain a list of records that (might) pass. If nothing in the table is processed using keys, the whole database is regarded as passing phase 2.
3. The final result list is built. If the entire expression is evaluated using keys, and the SORT= operand was not specified, phase 3 merely builds the result list.

   If the expression involved non-keyed fields, or certain operators (for example CONTAINS), all records on the list from phase 2 are read, and processed against the action table, to determine whether they pass.

   If the request included a SORT field, phase 3 will build a sort list for all records that pass. This list contains the sort fields for each record that passes.

4. If SORT was requested, the sort is performed, and the final result list is built.

Note: Optimization of the request can cause some of phase 2, 3, or 4 to be bypassed. For example, if there were no key fields in the scan, but the SORT key was the SEQUENCE key, the entire file would be read in that order, obviating the need for the actual SORT phase.

If the scan completes, and at least one record is found, &NDBRC will be set to 0, and the NCL variables will be set.

If the scan terminates without finding any records, &NDBRC will be set to 4,and the NCL variables will be set.

If the scan terminates because it exceeds the I/O, TIME, STORAGE or RECORD limits set (explicitly on the statement, or implicitly by the SYSPARMS NDBxxx settings), response codes 5 (I/O), 6 (TIME), 7 (STORAGE), or 8 (RECORD) will be set, and the NCL variables will be set, as indicated on page 2-432. In this case, the system variable &NDBRID might or might not be set to the first RID that the scan passed (not necessarily the lowest valued sort key record, if SORT).

If SEQUENCE is specified, the result-list-name is only saved if the scan completes and &NDBRC is set to 0.

The following NCL variables are set by a scan that terminates successfully, or unsuccessfully with a warning response:

&NDBSCANNRECS

Contains the number of records that passed the scan. When the scan terminates with a limit exceeded warning, this value might be non-zero if the scan terminated in phase 3 or later. If it is non-zero, the system variable &NDBRID will contain the first RID that passed the scan.

&NDBSCANIOCNT

Contains the number of logical I/Os performed by the scan. If the scan terminated on an I/O limit, this count will be 1 greater than that limit.

&NDBSCANTIME

Contains the elapsed time taken by the scan, accurate to one hundredth of a second, in the format SECONDS.TH. If the scan terminated on a TIME limit, the value will be marginally greater than that limit.

&NDBSCANSTG

Contains the maximum working storage used by the scan, expressed in Kilobytes. This working storage is used to hold intermediate results lists, sort fields, and so on. If the scan terminated on a STORAGE limit, this value will be marginally greater that the limit.