PostgreSQL 10 High Performance - Third Edition

cursor_tuple_fraction

When you start a query using a cursor instead of a regular statement, PostgreSQL doesn't know for sure how many rows you'll then retrieve. To lean towards both the possibility that you will only access a subset of them and the fact that cursor-based programs tend to be sensitive to latency, cursor_tuple_fraction allows you to lower the expected number of rows that cursor originated queries are expected to return. If set to 1.0, cursors will work the same as regular queries. At its default of 0.1, the query optimizer biases toward query plans that quickly return rows, assuming that only 10% of the total rows will be requested. This works similar to when a LIMIT is used on a query.