When a user issues a query, the parsed query is presented to a query optimizer, which uses information about how the data is stored to produce an efficient execution plan for evaluating the query.
An execution plan is a blueprint for evaluating a query, and is usually represented as a tree of relational operators (with annotations that contain additional detailed information about which access methods to use, etc).
Files and Access Methods layer includes a variety of software for supporting the concept of a file, which, in DBMS, is a collection of pages or a collections of records. This layer typically supports a heap file, or file of unordered pages, as well as indexes.
Buffer manager brings pages in from disk to main memory as needed in response to read requests.
The lowest layer of the DBMS software deals with management of space on disk, where the data is stored. Higher layers allocate, deallocate, read, and write pages through the disk space manager.
DBMS components associated with concurrency control and recovery include:
transaction manager, which ensures that transactions request and release locks according to a suitable locking protocol and schedules the execution transactions;
lock manager, which keeps track of requests for locks and grants locks on database objects when they become available
recover manager, which is responsible for maintaining a log, and restoring the system to a consistent state after a crash.