Compress Selected Columns
JMP lets you compress columns in a data table to minimize the size of the file and reduce the amount of memory required to analyze data. This feature is helpful when numeric columns contain many small integers or when any column contains fewer than 255 unique values. For example, compressing columns in a data table with 389 columns and 85,000 rows might decrease the file size from 250MB to 33MB, depending on the type of data.
When you compress columns, JMP verifies whether the data can be stored in a more compact form based on the data type:
• In character columns with fewer than 255 unique values, the List Check property is added to the column where appropriate (Figure 4.34).
The List Check property restricts the values in the selected column to valid values. The List Check property is not applied when the number of values in the selected column is too great. For example, if the number of values is almost the same as the number of rows, the data table does not add the List Check property to the column.
• For numeric columns, only those with the Best, Fixed Dec, or Data format are compressed. Data is compressed to 1-byte, 2-byte, or 4-byte integers when possible (Figure 4.35). For more information about short integers, see The Short-Integer Format in the The Column Info Window section.
A numeric column with non-integer values can also be compressed if there are fewer than 255 unique values. In this case, the List Check property is added to the column.
Caution: In a column with the List Check property, you can enter only a value that is in the list. Otherwise, JMP warns that the cell contains invalid data when you try to enter the new value. See List Check.
Figure 4.34 List Check Property Added to a Compressed Character Column
Figure 4.35 Column Info Window Showing Numeric Column before and after Compression
To compress columns, select one or more columns and select Cols > Utilities > Compress Selected Columns. (Select all columns if you do not know which columns can be compressed.)
The column or columns are compressed if possible. The log shows which columns were compressed and how they were compressed. (Select View > Log to show the log.)
Note: To compress a numeric column manually, set your Tables preferences to allow short numeric data and then change the column’s data type to 1-byte integer, 2-byte integer, or 4-byte integer. For more information about this preference, see Tables in the JMP Preferences section.