User Guide

61
Data Files
Creating a Data Cache
Although the virtual active file can vastly reduce the amount of temporary disk space
required, the absence of a temporary copy of the “active” file means that the original
data source
has to be reread for each procedure. For large data files read from an
external source, creating a temporary copy of the data may improve performance. For
example, for data tables read from a database source, the SQL query that reads the
informati
on from the database must be reexecuted for any command or procedure that
needs to read the data. Since virtually all statistical analysis procedures and charting
procedures need to read the data, the SQL query is reexecuted for each procedure
you run, wh
ich can result in a significant increase in processing time if you run a
large number of procedures.
If you have sufficient disk space on the computer performing the analysis (either
your loca
l computer or a remote server), you can eliminate multiple SQL queries and
improve processing time by creating a data cache of the active file. The data cache is
a temporary copy of the complete data.
Note:Byd
efault, the Database Wizard automatically creates a data cache, but if you
use the
GET DATA command in command syntax to read a database, a data cache
is not automatically created. (Command syntax is not available with the Student
Ver sio n
.)
To Create a Data Cache
E From the menus choose:
File
Cache Data...
E
Click OK or Cache Now.
OK creates a data cache the next time the program reads the data (for example, the
next ti
me you run a statistical procedure), which is usually what you want because
it doesn’t require an extra data pass.
Cache Now creates a data cache immediately,