Query-Evaluation Plan in SQL

A query evaluation plan, also known as an execution plan, is a set of steps that the database management system (DBMS) uses to execute a SQL query. The plan outlines the most efficient way to retrieve the requested data from the database.

There are several methods that a DBMS can use to generate a query evaluation plan. Here are some of the most common ones:

• Full Table Scan: This method involves scanning the entire table to retrieve the requested data. It is typically used when there are no indexes available to speed up the query. For example, the following query would use a full table scan:

   SELECT * FROM customers WHERE age > 30;

• Index Scan: This method involves using an index to retrieve the requested data. It is typically faster than a full table scan because the DBMS only needs to access the relevant index entries, rather than scanning the entire table. For example, the following query would use an index scan:

   SELECT * FROM customers WHERE last_name = 'Smith';

• Index Seek: This method is similar to an index scan, but it is even faster because the DBMS can use the index to directly locate the requested data, rather than scanning through the index entries. For example, the following query would use an index seek:

   SELECT * FROM customers WHERE customer_id = 12345;

• Nested Loop Join: This method involves joining two tables by iterating through one table and looking up matching rows in the other table. It is typically used when one of the tables is small and can fit into memory. For example, the following query would use a nested loop join:

   SELECT * FROM customers JOIN orders ON customers.customer_id = orders.customer_id;

• Hash Join: This method involves joining two tables by hashing the join columns and then comparing the hash values. It is typically used when both tables are large and cannot fit into memory. For example, the following query would use a hash join:

   SELECT * FROM customers JOIN orders ON customers.customer_id = orders.customer_id;

The DBMS will choose the most efficient query evaluation plan based on the available indexes, table statistics, and other factors. It is important for developers to understand how the DBMS generates query evaluation plans so that they can optimize their queries for better performance.