Database glossary: Freelancer level
43 Database terms at the Freelancer level, each with a short definition and a link to its full page.
Updated
- Attribute
- A named characteristic describing an entity or relation.
- Attribute Closure
- The attributes functionally determined by an attribute set under known dependencies.
- Cardinality
- The number of rows in a relation or the allowed counts in a relationship, depending on context.
- Cost-Based Optimization
- Choosing a plan by comparing estimated resource costs of alternatives.
- Cursor
- Steps through query results one row at a time; usually a last resort.
- Deadlock
- A cycle where transactions wait indefinitely for resources held by one another.
- Deferrable Constraint
- A constraint whose checking may be postponed until transaction completion.
- Dependency Preservation
- A decomposition property allowing original dependencies to be enforced without joining relations.
- Dirty Read
- Reading a change made by another transaction before that transaction commits.
- Entity
- A distinguishable thing or concept about which data is stored.
- Entity-Relationship Model
- A conceptual model describing entities, attributes, and relationships.
- Exclusive Lock
- A lock preventing other transactions from taking conflicting read or write locks.
- Execution Plan
- The physical operators and access paths used to execute a statement.
- Fifth Normal Form (5NF)
- A normal form where every nontrivial join dependency follows from candidate keys.
- Fourth Normal Form (4NF)
- BCNF with no nontrivial multivalued dependency unless its determinant is a superkey.
- Generalization
- Combining similar entity types into a broader supertype.
- Hash Join
- A join algorithm that builds a hash table for one input and probes it with the other.
- Identifying Relationship
- A relationship where a parent's key contributes to a dependent entity's identity.
- Index Scan
- Reading multiple entries from an index, often across a range or the full index.
- Index Seek
- Using index keys to navigate directly to a matching value or range.
- Initially Deferred
- A constraint mode starting each transaction with checking postponed until commit.
- Initially Immediate
- A constraint mode starting each transaction with checking performed after each statement.
- Join Dependency
- A constraint stating that a relation can be reconstructed by joining specified projections.
- Lock
- A concurrency-control claim restricting conflicting access to a database resource.
- Lock Granularity
- The size of the protected resource, such as a row, page, or table.
- Lossless Decomposition
- Splitting a relation so joining the parts recreates exactly the original relation.
- Lost Update
- A concurrency problem where one write unintentionally overwrites another transaction's write.
- Merge Join
- A join algorithm that advances through inputs ordered by their join keys.
- Minimal Cover
- An equivalent, simplified set of functional dependencies with no unnecessary parts.
- Multivalued Dependency
- A dependency where one attribute set determines independent sets of values for another.
- Nested Loop Join
- A join algorithm that searches the inner input for each qualifying outer row.
- Non-Repeatable Read
- Getting a different value when rereading a row changed and committed by another transaction.
- Optionality
- Whether participation in a relationship is required or optional.
- Phantom Read
- A repeated predicate query returning added or removed rows due to another committed transaction.
- Query Optimizer
- The RDBMS component that chooses a physical strategy for executing a query.
- Query Plan
- A representation of the operations selected or considered for a query.
- Sargable Predicate
- A predicate shaped so an optimizer can use an index search boundary.
- Selectivity
- The fraction of rows expected to satisfy a predicate.
- Shared Lock
- A lock allowing compatible readers while blocking conflicting writers.
- Specialization
- Dividing a broad entity type into narrower subtypes.
- Statistics
- Stored summaries of data distribution used to estimate query costs.
- Table Scan
- Reading a table broadly rather than locating rows through a selective index path.
- Weak Entity
- An entity that depends on another entity for part of its identity.