Why is Postgres using seq scan instead of index?

A lot of the times statistics are not updated on a table and it may not be possible to do so due to constraints. In this case, the optimizer will not know how many rows it should take in year > 2019. Thus it selects a sequential scan in lieu of full knowledge.

Is sequential scan bad?

A sequential scan is therefore not always bad – there are use cases, where a sequential scan is actually perfect. Still: Keep in mind that scanning large tables sequentially too often will take its toll at some point.

How do I stop sequential scan Postgres?

To prevent Postgres using an index only scan we select more columns than the index contains. The index scan is much faster. The next step is to look at the the query by changing the condition to match all rows. An index scan is still faster, but the percentage of the difference is far smaller.

What is sequential scan Postgres?

Of the three main ways Postgres has fetching rows from a table, a sequential scan is the most basic. To execute a sequential scan, Postgres literally iterates through a table a row at a time and returns the rows requested in the query.

Why is PostgreSQL not using index?

As we saw above, running a couple of queries on our posts table reveals that even given an index to use, Postgres will not always choose to use it. The reason why this is the case is that indexes have a cost to create and maintain (on writes) and use (on reads).

What is a seq scan?

A full table scan (also known as a sequential scan) is a scan made on a database where each row of the table is read in a sequential (serial) order and the columns encountered are checked for the validity of a condition.

Why the sequential scan in primary index is efficient?

The number of rows retrieved from the table may vary based on the particular constant values the query retrieves. When this happens, a sequential scan is actually most likely much faster than an index scan, so the query planner has in fact correctly judged that the cost of performing the query that way is lower.

What is index scan and sequential scan?

Seq Scan. The Seq Scan operation scans the entire relation (table) as stored on disk (like TABLE ACCESS FULL ). Index Scan. The Index Scan performs a B-tree traversal, walks through the leaf nodes to find all matching entries, and fetches the corresponding table data.

Why is Postgres not using my index?

What is an index scan?

An index scan occurs when the database manager accesses an index to narrow the set of qualifying rows (by scanning the rows in a specified range of the index) before accessing the base table; to order the output; or to retrieve the requested column data directly ( index-only access ).