There are two major types of query optimizers in relational databases: syntax-based and cost-based.
A syntax-based query optimizer creates a procedural plan for obtaining the answer to an SQL query, but the particular plan it chooses is dependent on the exact syntax of the query and the order of the clauses within the query. A syntax-based query optimizer executes the same plan every time, regardless of whether the number or composition of records in the database changes over time. Unlike a cost-based query optimizer, it neither maintains nor considers statistics about the database.
A cost-based query optimizer chooses among alternative plans to answer an SQL query. Selection is based on cost estimates for different plans. The factors in making cost estimates include the number of I/O operations, the amount of CPU time, and so on. A cost-based query optimizer estimates these costs by keeping statistics about the number and composition of records in a table or index and is not dependent on the exact syntax of the query or the order of clauses within the query (unlike a syntax-based query optimizer).