View (database) - Equivalence

Equivalence

A view is equivalent to its source query. When queries are run against views, the query is modified. For example, if there exists a view named accounts_view with the content as follows:

accounts_view: ------------- SELECT name, money_received, money_sent, (money_received - money_sent) AS balance, address, ... FROM table_customers c JOIN accounts_table a ON a.customer_id = c.customer_id

then the application could run a simple query such as:

Simple query ------------ SELECT name, balance FROM accounts_view

The RDBMS then takes the simple query, replaces the equivalent view, then sends the following to the query optimizer:

Preprocessed query: ------------------ SELECT name, balance FROM (SELECT name, money_received, money_sent, (money_received - money_sent) AS balance, address, ... FROM table_customers c JOIN accounts_table a ON a.customer_id = c.customer_id )

From this point on the optimizer takes the query, removes unnecessary complexity (for example: it is not necessary to read the address, since the parent invocation does not make use of it) and then sends the query to the SQL engine for processing.