Which SELECT statement will always access the database when table A and table B are partially buffered?

The chosen statement is correct because it involves a join operation between two partially buffered tables, table A and table B. When a join is executed on two tables, the database management system must access both tables in order to produce the result set, whether or not the necessary data is available in the buffer. Since table A and table B are both partially buffered, there is a high chance that the relevant data for at least one of the tables may not be in memory, necessitating a call to the database for that data. Therefore, this select statement guarantees a database access as it retrieves data that relies on the combination of fields from both tables.

The other statements do not necessarily guarantee database access under the same conditions. For example, the SELECT SINGLE statement retrieves a single record from the database and can often be served from the buffer if the data is already cached, hence it may not always access the database. Additionally, using CLIENT SPECIFIED or FOR UPDATE in certain contexts can also provide optimizations that rely on existing data in the buffer. Thus, while they can access the database, they do not make it a certainty like the join statement does.