PostgreSQL makes its decisions about how queries execute based on statistics collected about each table in your database. This information is collected by analyzing the tables, either with the ANALYZE statement or via autovacuum doing that step. In either case, the amount of information collected during the analyze step is set by default_statistics_target. Increasing this value makes analysis take longer, and as analysis of autovacuum happens regularly, this turns into increased background overhead for database maintenance. But, if there aren't enough statistics about a table, you can get bad plans for queries against it.
The default value for this setting used to be very low (that is, 10), but was increased to 100 ...
