4767e3a6___Documentation__8_4__pg_stats__Description
[Name] n_distinct [Type] real [Description] 
If greater than zero, the estimated number of distinct values in the column. If less than zero, the negative of the number of distinct values divided by the number of rows. (The negated form is used when ANALYZE believes that the number of distinct values is likely to increase as the table grows; the positive form is used when the column seems to have a fixed number of possible values.) For example, -1 indicates a unique column in which the number of distinct values is the same as the number of rows
PostgreSQL: Documentation: 8.4: pg_stats