Does the number of columns affect query performance?

Question

CASE 1: I have a table with 30 columns and I query using 4 columns in the where clause.

CASE 2: I have a table with 6 columns and I query using 4 columns in the whe

Answer 1

Since you specified you are using the WHERE clause it will depend on how many rows are returned. If the value in your WHERE clause is UNIQUE or a PRIMARY KEY than the difference is almost non-existent. You can use EXPLAIN ANALYZE in front of your SELECT statement to view the planning time and execution time values and than you can compare your queries.