Single Precision

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Single Precision Number Fields

One way that we find that we can reduce the size of a database is by changing the precision of fields from double to single. In some cases changing all the double precision number fields in a table from double to single real can trim the space used by about 40%. This can make some difference to performance in reading the file. You can store numbers from -3.402823E38 to -1.401298E-45.

Basically if the number is going to have over 6 significant figures in it, then think very carefully about this change as those precise number will get trimmed. An export of the table to a text file will allow you recover if you do trim a number incorrectly. Numbers such as 25.773 and -43000.1 don't need double precision. Don't forget to compact your database after this doing this.

Warning: Occasionally trimming data can cause problems if you rely on double precision functions like Avg in consolidated queries like this SQL statement

select Avg(field) from tableName;

In this case the results will include annoying errors on the last significant figure that require you to add formatting statement to your reports and queries and do other little tricks when exporting to Excel.

Written by Garry Robinson from GR-FX in Sydney, Australia