Introduction

Mini SQL 2.0 includes internal support for system variables (often known as pseudo fields or

pseudo columns). These variables can be accessed in the same way that normal table fields are

accessed although the information is provided by the database engine itself rather than being

loaded from a database table. System variables are used to provide access to server maintained

information or meta data relating to the databases.

System variables may be identified by a leading underscore in the variables name. Such an

identifier is not valid in mSQL for table or field names. Examples of the supported system

variables and uses for those variables are provided below.

Available System Variables

The mSQL 2 engine currently supports the following system variables:

_rowid

G   

The _rowid system variable provides a unique row identifier for any row in a table. The

value contained in this variable is the internal record number used by the mSQL engine

to access the table row. It may be included in any query to uniquely identify a row in a

table. An example of such queries could be :

select _rowid, first_name, last_name from emp_details

H   

where last_name =  Smith 

update emp_details set title =  IT Manager 

where _rowid = 57

The candidate row module is capable of utilising 

_rowid

 values to increase the

performance of the database. In the second example query above, only 1 row

(the row with the internal record ID of 57) would be accessed. This is in

contrast to a sequential search through the database looking for that value which

may result in only 1 row being modified but every row being accessed. Using

the _rowid value to constrain a search is the fastest access method available in

mSQL 2.0. As with all internal access decisions, the decision to base the table

access on the _rowid value is automatic and requires no action by the

programmer or user other than including the _rowid variable in the 

where

clause of the query.

_timestamp

The _timestamp system variable contains the time at which a row was last modified.

The value, although specified in the standard UNIX time format (i.e. seconds since the

epoch), is not intended for interpretation by application software. The value is intended

to be used as a point of reference via which an application may determine if a particular

row has was modified before or after another table row. The application should not try

to determine an actual time from this value as the internal representation used may

change in a future release of mSQL.