Analysing a query-plan PostgreSQL Notes Prev Chapter 12. A query is running much slower than you expected. You suspect an index is being ignored or you are missing an index on a particular column. It shows the decisions it makes and the costs it calculated that led to those decisions. The rows are an estimate on the number of rows being returned and the width represents the amount of data involved per record.
Complex queries produce complex query plans - pay careful attention to the rows part.
If Postgres thinks it will fetch 1000 rows from a 1500 row table then it will ignore the index and use a Seq Scan instead. If it estimates the rows wrong it will make the wrong decision.