Once you know that a table is not currently being used, the next question is, When was it last used?
PostgreSQL does not have any built-in last used information about tables, so you have to use other means to figure it out.
If you have set up a cron job to collect usage statistics, as described in the previous chapter, then it is relatively easy to find out the last date of change using a SQL query.
Other than this, there are basically two possibilities, neither of which give you absolutely reliable answers.
You can either look at the actual timestamps of the files in which the data is stored, or you can use the xmin and xmax system columns to find out the latest transaction ID that changed the table data.
In this recipe, we will cover the first case and focus on the date information in the table's files.
The following PL/pgSQL function looks for the table's data files to get the value...