4767e3a6___Documentation__8_4__pg_stats__Description
[Name] correlation [Type] real [Description] 
Statistical correlation between physical row ordering and logical ordering of the column values. This ranges from -1 to +1. When the value is near -1 or +1, an index scan on the column will be estimated to be cheaper than when it is near zero, due to reduction of random access to the disk. (This column is NULL if the column data type does not have a < operator.)
PostgreSQL: Documentation: 8.4: pg_stats