CSE 344 Section 6

Transactions

BCNF

Boyce-Codd Normal Form (BCNF)

A relation R is in BCNF if every set of attributes in R is either a superkey or its closure is the same set (trivial FD).

*that is, either {a}+ -> a OR {a}+ -> {all cols}, where a is any col in the table

• BCNF reduces redundancy at the expense of creating additional tables.
• BCNF may have several correct decompositions.
• Some functional dependencies may be lost.

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Boyce-Codd Normal Form (BCNF)

Not BCNF! SSN is not a trivial FD or a superkey

BCNF Decomposition

Algorithm

C = the set of all attributes in the relation R

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Look for an attribute (or a set of attributes) that meets the following conditions (non-BCNF flag!):

• X⁺ != X
• X⁺ != C

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If such attribute exists, it is a violation of the BCNF condition; decompose R:

• R1 = X⁺
• R2 = (C - X⁺) union (X)
• Normalize each table (recursive call)

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Else, the table is in BCNF

BCNF

Example

Relation: R (A, B, C, D, E)

FDs:

• A → E
• BC → A
• DE → B

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Goal: Decompose R into BCNF

BCNF

Example

R (A, B, C, D, E)

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A⁺ = {A, E}

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BCNF compliant

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BCNF compliant

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BCNF compliant

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{B, C}⁺ = {B, C, A}

BCNF

Example

Notice that {A}⁺ = {A,E}, which violates the BCNF condition

• We split R to R1(A,E) and R2(A,B,C,D)
• R1 satisfies BCNF
• R2 does not satisfy BCNF because: {B,C}⁺ = {B,C,A}

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{B,C}⁺ = {B,C,A} violates the BCNF condition

• *Notice there is no E in R2 so we don't need to consider DE → B
• Split R2 to: R21(B,C,A) and R22(B,C,D)
• R21 and R21 satisfy BCNF now

Transactions

ACID

Atomic�Consistent�Isolated�Durable

Everything a concurrent database user wants…

How do ACID properties help us?

• Ensure that a transaction maintains database integrity
• ACID compliance matters to important databases ($$$)
• In this section: delve into the DB Internals required to guarantee ACID

Atomicity

What? “Atom” - a transaction is indivisible; data operations inside the transaction work in an all or nothing fashion

How? Transaction commit and rollback

Example: Transfer of funds from one account to another involves an atomic transaction that includes operations of debit and credit

Consistency

What? Only data operations that comply with database validity constraints are allowed.

Once a transaction is completed, it must not leave the database incomplete or inconsistent.

How? Enforcement of consistency rules upon data entry/updation

Example: A database having both first and last name fields will not accept if you enter only one of those - must enter both for the transaction to be complete

Isolation

What? The appearance of a transaction happening by itself irrespective of what other transactions are happening concurrently

How? User views the database that was present right before the view request is made and any further transactions that are in process after it are ignored

Example: A teller looking up a balance must be isolated from a concurrent transaction involving a withdrawal from the same account

Durability

What? Once a transaction is complete the information as changed will survive failures of any kind

How? Creation of data “mirrors”

Example: Trying to make an update that leads to a system failure should not tamper with any of the previous data and transactions

Txn Perspective

A database is a collection of "elements" that can be written to or read from.

Operation - A read or write to an element (later: or an insert or delete)

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Example: What operation does the following SQL query contain?�SELECT cases FROM Country WHERE cname = ‘France’;

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Txn Perspective

A database is a collection of "elements" that can be written to or read from.

Operation - A read or write to an element (later: or an insert or delete)

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Example: What operation does the following SQL query contain?�SELECT cases FROM Country WHERE cname = ‘France’;

Answer: R(Country) or R(France), depending on whether the "element" is a table or a row (assuming cname is a key). Let's go with the finer-grained element.

Definitions

Operation -

Transaction -

Schedule -

Serial schedule -�

Serializable schedule - �

Definitions

Operation - read or write of an element (later: insert or delete)

Transaction - series of operations meant to have the ACID guarantees

Schedule - ordering of the operations of some txns

Serial schedule - schedule where each txn is executed one after another.� No interleaving

Serializable schedule - a schedule that is behaviorally equivalent � to a serial schedule

Practice - convert SQL to operations

Txn 1:

1a) x = SELECT cases FROM Country WHERE cname = ‘France’;

1b) x = x + 200;

1c) UPDATE Country SET cases = `x` WHERE cname = ‘France’;

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Txn 2:

2a) y = SELECT cases FROM Country WHERE cname = ‘Spain’;

2b) y = y / 5;

2c) z = SELECT cases FROM Country WHERE cname = ‘France’;

2d) UPDATE Country SET cases = `y + z` WHERE cname = ‘France’;

Practice - convert SQL to operations

Txn 1:

1a) x = SELECT cases FROM Country WHERE cname = ‘France’;

1b) x = x + 200;

1c) UPDATE Country SET cases = `x` WHERE cname = ‘France’;

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Txn 2:

2a) y = SELECT cases FROM Country WHERE cname = ‘Spain’;

2b) y = y / 5;

2c) z = SELECT cases FROM Country WHERE cname = ‘France’;

2d) UPDATE Country SET cases = `y + z` WHERE cname = ‘France’;

1a) R(France)

1b)

1c) W(France)

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2a) R(Spain)

2b)

2c) R(France)

2d) W(France)

Conflict Serializability

Definitions

Non-conflicting swap - �

Conflicting swap -

Conflict serializable schedule - �

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Definitions

Non-conflicting swap - a pair of adjacent operations in a schedule � that can be reversed without affecting the schedule's behavior

Conflicting swap - (any two ops in same txn), RW, WR, WW

Conflict serializable schedule - a schedule that can be transformed � into a serial schedule via a series of non-conflicting swaps

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Conflict Serializability

Checking for conflict serializability -> precedence graph and cycle checking

Conflict Serializability

Checking for conflict serializability -> precedence graph and cycle checking

Serializability

“Conflict serializable” is a stronger constraint than “serializable”

I.e. Any schedule that is conflict serializable must be serializable.

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Serializable

Conflict

Serializable

Easy to check

Not easy to check

Serializability

S1: w1(Y); w2(Y); w1(X); w2(X); w3(X)

S2: w1(Y); w2(Y); w2(X); w1(X); w3(X)

Are these serializable? Conflict serializable?

Serializability

S1: w1(Y); w2(Y); w1(X); w2(X); w3(X)

S2: w1(Y); w2(Y); w2(X); w1(X); w3(X)

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Conflict Serializable

Serializability

S1: w1(Y); w2(Y); w1(X); w2(X); w3(X)

S2: w1(Y); w2(Y); w2(X); w1(X); w3(X)

Conflict Serializable

Not Conflict Serializable, but Serializable (depending on our definition of write)

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Worksheet