Logging

Physiological Logging

Derby implements a combination of physical and logical logging. For actions on the same page, it uses physical logging. For BTree operations, which might affect several pages, it uses physical redo and logical undo.

Query Interface

SQL

Query Execution

Materialized Model

Subqueries can only be materialized if they not correlate with outer queries, and return one row. For subqueries that cannot be flattened (DISTINCT), optimization can be made on subqueries such as using Hash Join.

Storage Model

N-ary Storage Model (Row/Record)

Foreign Keys

Supported

Foreign key is implemented as one of the CONSTAINT clauses. There are two levels of CONSTAINTS, column level and table level. Foreign key constraint in a column level enforces that the values in the column must corresponds to the values in the referenced column marked as primary key or unique key. Table level constraint works similarly, but it is for multiple columns.

Insert, update or delete instructions will be rejected with a statement exception if the foreign key constraint is violated. The constraint check can be at statement execution or commit depending on the constraint mode. (IMMEDIATE or DEFERRED).

Data Model

Relational

Stored Procedures

Supported

Derby supports prepared statements with parameter substitutions.

Checkpoints

Non-Blocking Fuzzy

Derby implements ARIES with slight variance. It allows fuzzy checkpointing. However, unlike ARIES implementation, it does not store a dirty page list in checkpoint records. Instead, during checkpoint, Derby flushes all database pages to disk. The checkpoint control files contains "undoLWM" and "RedoLWM". "UndoLWM" is set as the the starting LSN of the oldest active transaction when checkpoint starts, and "RedoLWM" is the the current LSN of the checkpoint.

Storage Organization

Heaps

Concurrency Control

Not Supported

There is no concurrency control logic implemented. It depends on the correct application logics to prevent deadlocks. Deadlock detection is implemented by Derby such that the transaction that holds the least number of locks will be aborted.

Storage Architecture

Hybrid

Derby mainly support on-disk database. It also provides in-memory database for testing and developing applications. Note that in-memory database do not show in the Derby system directories.

Joins

Nested Loop Join Hash Join

Derby provides two types of join strategies -- nested loop and hash join. Nested loop join is more preferable in most cases. Hash join is preferred when inner table values are unique and outer table have many qualifying rows. Also, when the system estimated that the amount of memory required for hash join exceeds the amount available, nested loop will be used.

Views

Virtual Views

Indexes

B+Tree

Derby implements standard B+ Tree algorithms. It stores keys in leaf pages only. The B+ Tree supports page-level latching. Derby uses exclusive latches, not shared latches.

Isolation Levels

Read Uncommitted Read Committed Serializable Repeatable Read

It supports all four level of isolations. Isolation levels only differ for SELECT statements. They behave the same for other operations.