In this post, I am sharing few important parameters for configuring the Query planner of PostgreSQL.

I found this help on official PostgreSQL page, so I am sharing from there.
Reference link:

For the Query optimization, these parameters are very important, so I copied and sharing same information from the official page.

Made list of few important parameters:

These configuration parameters provide a crude method of influencing the query plans chosen by the query optimizer.

If the default plan chosen by the optimizer for a particular query is not optimal, a temporary solution is to use one of these configuration parameters to force the optimizer to choose a different plan.

enable_indexscan (boolean): Enables or disables the query planner’s use of index-scan plan types. The default is on

enable_hashjoin (boolean): Enables or disables the query planner’s use of hash-join plan types. The default is on

enable_hashagg (boolean): Enables or disables the query planner’s use of hashed aggregation plan types. The default is on

enable_mergejoin (boolean): Enables or disables the query planner’s use of merge-join plan types. The default is on

enable_nestloop (boolean): Enables or disables the query planner’s use of nested-loop join plans. It is impossible to suppress nested-loop joins entirely, but turning this variable off discourages the planner from using one if there are other methods available. The default is on

enable_seqscan (boolean): Enables or disables the query planner’s use of sequential scan plan types. It is impossible to suppress sequential scans entirely, but turning this variable off discourages the planner from using one if there are other methods available. The default is on

enable_sort (boolean): Enables or disables the query planner’s use of explicit sort steps. It is impossible to suppress explicit sorts entirely, but turning this variable off discourages the planner from using one if there are other methods available. The default is on

Every day If you are doing performance optimization related stuff, sometime you also confuse on the decision of the Query Planner because It might be choose a wrong plan.

The SQL Server and MySQL provide features like: Index force or Index hint for SQL query, but PostgreSQL does not provide such feature because Once you set Index hint on the query, in future data is increasing and your Index hint might be not working accordingly.

Database Theory: Table Scan vs Index Scan vs Index Seek

In the PostgreSQL, Query Planner takes the decision base on the updated statistics and choose the Table Scan or Index Scan.

For smaller table, sequential scan is always faster compare to Index scanning. But sometimes what I have found is: due to invalid parameter PostgreSQL skips the indexes and start the table scanning which leads to poor performance.

In this situation, we should disable the Sequential Scan option so that Query Planner force to use available Indexes and execute the Query.

In PostgreSQL, We cannot enable / disable Index Scan or Table Scan at individual query level.

PostgreSQL provides two main parameters for Index Scan or Table Scan:
By default, this both parameters are ON, but as per the your requirement you can change the value.
You can find these parameters in postgresql.conf file.

enable_seqscan
enable_indexscan

You should never make the force on Query Planner because Forcing is a very bad idea. It might be useful to check if it will be faster, but production code should never use such tricks.