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The ACID Properties AND Normal Forms

The ACID Properties AND Normal Forms. ACID Transactions. Atomic: Either all of a transaction or None of it affects the database Consistent: When a transaction ends, the database obeys all constraints

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The ACID Properties AND Normal Forms

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  1. The ACID PropertiesANDNormal Forms

  2. ACID Transactions • Atomic: Either all of a transaction or None of it affects the database • Consistent: When a transaction ends, the database obeys all constraints • Isolated: Two running transactions cannot pass values to each other, via the database or other data store • Durable: Once a transaction has “committed”, its updates are permanent

  3. Atomicity • Use a local log to store a transaction’s partial result • If a transaction does something illegal, toss out the log

  4. Consistent • Check constraints in phase 1 • Some are immediate, like domains • Others don’t have to be true until the commit point, like FKs

  5. Isolated • Transactions commit in a linear order • Serializability is enforced • Results become available only after atomic commit point

  6. Durable • Database has one state and it is in nonvolatile storage • Keep checkpoints and transaction logs

  7. Deadlock • Loops of transactions wait on each other • Detection: use time-outs • Prevention: use “waits for” graph

  8. The DB Design Process • Start with an entity model • Map to tables • Create PKs and FKs • Create other constraints • Normalize tables

  9. Our focus: normalization • Goals • Minimize redundant data • Minimize “update anomalies”

  10. Functional and Multivalued Dependencies • FD  • We say that ai FD-> aj • Or “ai functionally determines aj” • MVD-> • We say that ai MVD-> aj • Or “ai multivalued determines aj” • Note: the right side of an FD or an MVD can be a set of attributes

  11. 3rd normal form • Third (3NF) Every non-key column must depend only on the primary key.

  12. NF3 fixed and NF4 • Boyce-Codd (BCNF) A non-key column can’t be dependent on another non-key column. • Fourth (4NF) A table must not have more than one multivalued dependency, where the primary key has a one-to-many relationship to non-key columns.

  13. 3NF: remove transitive dependencies Customer ID Address ZIP 18 112 First 80304 17 123 Ash 80303 16 123 Ash 80303

  14. 3NF, continued Break into two tables: Customer ID Address Address Zip

  15. Fourth NormalForm SSN PhoneN ChildSSN 111111 123-4444 222222 111111 123-4444 333333 111111 321-5555 222222 111111 321-5555 333333 222222 987-6666 444444 222222 777-7777 444444 222222 987-6666 555555 222222 777-7777 555555 Person redundancy • Relation has redundant data • Yet it is in BCNF (since there are no non-trivial FDs) • Redundancy is due to set valued attributes (in the E-R sense), not because of the FDs

  16. 4NF: Separate pairs of MVDs Mothers_PhoneFathers_PhoneChild_Name Break into: Adult Phone joe 1 joe 2 sue 3 sue 4 And Adult Child joe Tommy joe Bobby sue Tommy sue Bobby Note: both fields needed for PK

  17. Tradeoffs • “Decomposition” makes it harder to misunderstand the database schema • But Decomposition create narrow tables that might not correspond to forms in the real world • And Decomposition leads to extra joins • One solution is to pre-join data

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