chapter 10

Every transaction has certain characteristics attributed to it. These characteristics are specified by a SET TRANSACTION statement in SQL. The characteristics are the access mode, the diagnostic area size, and the isolation level. The isolation level option is specified using the statement ISOLATION LEVEL, where the value can be READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, or SERIALIZABLE select all Possible Violations Based on Isolation Levels as Defined in SQL READ UNCOMMITTED A) Phantoms B) Nonrepeatable read: C) Dirty read D) none of the above

1) dirty read, 2) unrepeatable read, 3) phantoms

The isolation level option is specified using the statement ISOLATION LEVEL, where the value can be READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, or SERIALIZABLE. The use of the term SERIALIZABLE here is based on not allowing violations that cause: A) dirty read, phantoms, messy reads B) unrepeatable read, dirty read, phantoms C) phantoms, repeatable read, dirty read D) dirty read, unrepeatable read, phantoms

1) dirty read, 2) unrepeatable read, 3) phantoms

Every transaction has certain characteristics attributed to it. These characteristics are specified by a SET TRANSACTION statement in SQL. The characteristics are the access mode, the diagnostic area size, and the isolation level. The isolation level option is specified using the statement ISOLATION LEVEL, where the value can be READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, or SERIALIZABLE select all Possible Violations Based on Isolation Levels as Defined in SQL READ COMMITTED A) Phantoms B) Nonrepeatable read: C) Dirty read D) none of the above

2) unrepeatable read, 3) phantoms

Every transaction has certain characteristics attributed to it. These characteristics are specified by a SET TRANSACTION statement in SQL. The characteristics are the access mode, the diagnostic area size, and the isolation level. The isolation level option is specified using the statement ISOLATION LEVEL, where the value can be READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, or SERIALIZABLE select all Possible Violations Based on Isolation Levels as Defined in SQL REPEATABLE READ A) Phantoms B) Nonrepeatable read: C) Dirty read D) none of the above

3) phantoms

A transaction is an atomic unit of work that should either be completed in its entirety or not done at all. For recovery purposes, the system needs to keep track of when each transaction starts, terminates, and commits, or aborts. Therefore, the recovery manager of the DBMS needs to keep track of the following operations __ This signals a successful end of the transaction so that any changes (updates) executed by the transaction can be safely committed to the database and will not be undone. A) BEGIN_TRANSACTION: B) READ or WRITE C) END_TRANSACTION: D) COMMIT_TRANSACTION: E) ROLLBACK (or ABORT):

: COMMIT_TRANSACTION:

The transaction is a complete unit, and is executed in its entirety or not at all A) Atomicity B) Consistency C) Isolation D) Durability

Atomicity

Transactions are said to have the following four desirable properties, commonly referred to as ACID properties. A) Durability, Consistency, Atomicity, Isolation B) Atomicity, Isolation, Stability, Consistency C) Consistency, Atomicity, Durability, Reliability, D) Atomicity, Stability, Durability, Isolation,

Atomicity Consistency Isolation Durability

A transaction is an atomic unit of work that should either be completed in its entirety or not done at all. For recovery purposes, the system needs to keep track of when each transaction starts, terminates, and commits, or aborts. Therefore, the recovery manager of the DBMS needs to keep track of the following operations: __ This marks the beginning of transaction execution A) BEGIN_TRANSACTION: B) READ or WRITE C) END_TRANSACTION: D) COMMIT_TRANSACTION: E) ROLLBACK (or ABORT):

BEGIN_TRANSACTION:

With SQL every transaction must have an explicit end statement, which is either a __ or a __ A) READ WRITE B) READ ONLY or READ WRITE C) COMMIT or a ROLLBACK D) SET TRANSACTION

COMMIT or a ROLLBACK

A transaction can have either of two results. If it finishes successfully, the transaction will be saved with the database attaining a new consistent state. This is known as a __. Conversely, if there is a problem with the transaction and it does not complete successfully, it will be aborted. The formal term for this aborted transaction is a __, select all that apply: A) Commit B) Insert C) Rollback D) Aborted

Commit, Rollback

__ is the management of transactions that occur at the same time and is performed to ensure that the transactions do not interfere with one another. A) Correction control B) Accuracy control C) Concurrency control D) none of the above

Concurrency control

__ is the management of transactions that occur at the same time and is performed to ensure that the transactions do not interfere with one another. A) Timestamping B) transaction C) locking and timestamp D) Concurrency control E) lock F) stamping and sealing G) optimistic

Concurrency control

The transaction must change the database from one set state to another set state. A) Atomicity B) Consistency C) Isolation D) Durability

Consistency

characteristics are specified by a SET TRANSACTION statement in SQL. The characteristics are the access mode, the diagnostic area size, and the isolation level. The diagnostic area size option, __ n, specifies an integer value n, which indicates the number of conditions that can be held simultaneously A) DIAGNOSTIC SIZE B) isolation level. C) SERIALIZABLE D) READ COMMITTED

DIAGNOSTIC SIZE

The use of the term SERIALIZABLE here is based on not allowing violations that cause dirty read, unrepeatable read, and phantoms. If a transaction executes at a lower isolation level than SERIALIZABLE, then one or more of the following three violations may occur: __ A transaction T1 may read the update of a transaction T2, which has not yet committed. If T2 fails and is aborted, then T1 would have read a value that does not exist and is incorrect. A) Phantoms B) Nonrepeatable read: C) Dirty read D) none of the above

Dirty read:

Committed transactions are permanent and should not be lost owing to any ensuing malfunction A) Atomicity B) Consistency C) Isolation D) Durability

Durability

A transaction is an atomic unit of work that should either be completed in its entirety or not done at all. For recovery purposes, the system needs to keep track of when each transaction starts, terminates, and commits, or aborts. Therefore, the recovery manager of the DBMS needs to keep track of the following operations: __ This specifies that READ and WRITE transaction operations have ended and marks the end of transaction execution. However, at this point, it may be necessary to check whether the changes introduced by the transaction can be permanently applied to the database (committed) or whether the transaction has to be aborted because it violates serializability or for some other reason. A) BEGIN_TRANSACTION: B) READ or WRITE C) END_TRANSACTION: D) COMMIT_TRANSACTION: E) ROLLBACK (or ABORT):

END_TRANSACTION:

TRUE/FALSE If concurrent transactions only read data, a conflict exists and the order of execution is significant. If concurrent transactions read or write entirely different data structures, a conflict exists and the order of execution must be stated. If concurrent transactions read or write the same data structures, the order of execution is significant. list all errors

FALSE If concurrent transactions only read data, no conflict exists and the order of execution is not significant. If concurrent transactions read or write entirely different data structures, no conflict exists and the order of execution is not significant. If concurrent transactions read or write the same data structures, the order of execution is significant.

The transaction is independent from other transactions. An incomplete transaction should not be visible to other transactions A) Atomicity B) Consistency C) Isolation D) Durability

Isolation

The following are the main problems that concurrency control is designed to address. __ When simultaneous updates occur to a relation, one update may override another. A) Unrepeatable query results B) Uncommitted updates C) Lost updates D) none of the above

Lost updates

The use of the term SERIALIZABLE here is based on not allowing violations that cause dirty read, unrepeatable read, and phantoms. If a transaction executes at a lower isolation level than SERIALIZABLE, then one or more of the following three violations may occur __ A transaction T1 may read a given value from a table. If another transaction T2 later updates that value and T1 reads that value again, T1 will see a different value. A) Phantoms B) Nonrepeatable read: C) Dirty read D) none of the above

Nonrepeatable read:

How do you rollback a committed transaction? A) It is permanent and cannot be undo B) by executing another transaction C) by using the UNDO command in a SQL statement D) by using the ROLLBACK command in a SQL statement

Once a transaction has been committed, it cannot be rolled back. The only way to undo a committed transaction is to execute another transaction that reverses the changes made by the first transaction

The use of the term SERIALIZABLE here is based on not allowing violations that cause dirty read, unrepeatable read, and phantoms. If a transaction executes at a lower isolation level than SERIALIZABLE, then one or more of the following three violations may occur __ A transaction T1 may read a set of rows from a table, perhaps based on some condition specified in the SQL WHERE-clause. Now suppose that a transaction T2 inserts a new row r that also satisfies the WHERE-clause condition used in T1, into the table used by T1. The record R is this kind of record because it was not there when T1 starts but is there when T1 ends. A) Phantoms B) Nonrepeatable read: C) Dirty read D) none of the above

Phantoms:

The isolation level option is specified using the statement ISOLATION LEVEL, where the value can be READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, or SERIALIZABLE. The default isolation level is SERIALIZABLE , although some systems use __ as their default. A) DIAGNOSTIC SIZE B) isolation level. C) SERIALIZABLE D) READ COMMITTED

READ COMMITTED

characteristics are specified by a SET TRANSACTION statement in SQL. The characteristics are the access mode, the diagnostic area size, and the isolation level. The access mode can be specified as __ or __ A) READ WRITE B) READ ONLY or READ WRITE C) COMMIT or a ROLLBACK D) SET TRANSACTION

READ ONLY or READ WRITE

characteristics are specified by a SET TRANSACTION statement in SQL. The characteristics are the access mode, the diagnostic area size, and the isolation level. The access mode can be specified as READ ONLY or READ WRITE. The default is __, unless the isolation level of READ UNCOMMITTED is specified. A) READ WRITE B) READ ONLY or READ WRITE C) COMMIT or a ROLLBACK D) SET TRANSACTION

READ WRITE

A transaction goes into an active state immediately after it starts execution, where it can execute its __ operations. A) active state B) execution C) READ and WRITE D) partially committed

READ and WRITE

A transaction is an atomic unit of work that should either be completed in its entirety or not done at all. For recovery purposes, the system needs to keep track of when each transaction starts, terminates, and commits, or aborts. Therefore, the recovery manager of the DBMS needs to keep track of the following operations: __ These specify read or write operations on the database items that are executed as part of a transaction. A) BEGIN_TRANSACTION: B) READ or WRITE C) END_TRANSACTION: D) COMMIT_TRANSACTION: E) ROLLBACK (or ABORT):

READ or WRITE:

A transaction is an atomic unit of work that should either be completed in its entirety or not done at all. For recovery purposes, the system needs to keep track of when each transaction starts, terminates, and commits, or aborts. Therefore, the recovery manager of the DBMS needs to keep track of the following operations: __ This signals that the transaction has ended unsuccessfully so that any changes or effects that the transaction may have applied to the database must be undone. A) BEGIN_TRANSACTION: B) READ or WRITE C) END_TRANSACTION: D) COMMIT_TRANSACTION: E) ROLLBACK (or ABORT):

ROLLBACK (or ABORT):

Optimistic concurrency methods reduce the extra processing required by locking and timestamping techniques. Following is one of the three steps involved in optimistic concurrency control, depending on whether a transaction is a read or write operation. __: This step continues from the beginning of the transaction until directly before the commit. The transaction reads the data items needed and stores them in local variables. Any updates are applied to these copies, not to the actual data item. A) write B) read C) validate D) none of the above

Read

The isolation level option is specified using the statement ISOLATION LEVEL, where the value can be READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, or SERIALIZABLE. The default isolation level is __, although some systems use READ COMMITTED as their default. A) DIAGNOSTIC SIZE B) isolation level. C) SERIALIZABLE D) READ COMMITTED

SERIALIZABLE

characteristics are specified by a __ statement in SQL. The characteristics are the access mode, the diagnostic area size, and the isolation level. A) READ WRITE B) READ ONLY or READ WRITE C) COMMIT or a ROLLBACK D) SET TRANSACTION

SET TRANSACTION

The basic definition of an __ is similar to our already defined concept. That is, it is a logical unit of work and is guaranteed to be atomic. A) write B) read C) validate D) none of the above

SQL transaction

TRUE/FALSE If concurrent transactions only read data, no conflict exists and the order of execution is not significant. If concurrent transactions read or write entirely different data structures, no conflict exists and the order of execution is not significant. If concurrent transactions read or write the same data structures, the order of execution is significant..

TRUE If concurrent transactions only read data, no conflict exists and the order of execution is not significant. If concurrent transactions read or write entirely different data structures, no conflict exists and the order of execution is not significant. If concurrent transactions read or write the same data structures, the order of execution is significant.

__ orders all transactions so that the oldest transaction receives priority in case of a data access conflict A) Timestamping B) transaction C) locking and timestamp D) Concurrency control E) lock F) stamping and sealing G) optimistic

Timestamping

The following are the main problems that concurrency control is designed to address. __ problems can occur when one transaction is allowed to see the intermediate results of another transaction. A) Unrepeatable query results B) Uncommitted updates C) Lost updates D) none of the above

Uncommitted update

The following are the main problems that concurrency control is designed to address. __ result problems can occur if a database query accesses only partially updated data. This is also referred to as a dirty read. A) Unrepeatable query results B) Uncommitted updates C) Lost updates D) none of the above

Unrepeatable query results

Optimistic concurrency methods reduce the extra processing required by locking and timestamping techniques. Following is one of the three steps involved in optimistic concurrency control, depending on whether a transaction is a read or write operation __: After the read step, this step ensures that no conflicts will occur if the updates are made to the database. For transactions that only read a data item, a check is made to ensure that the data values read are still current. If no conflict is found, a commit is performed. For an update transaction, a check is made to ensure that the database will be left in a consistent state. If serializability cannot be preserved, the transaction is rolled back. A) write B) read C) validate D) none of the above

Validate

a transaction can go to the failed state 1) if one of the checks fails or 2) if the transaction is aborted during its active state The transaction may then have to be rolled back to undo the effect of its __ operations on the database. A) ROLLBACK B) READ C) END D) None of the above

WRITE operations

Optimistic concurrency methods reduce the extra processing required by locking and timestamping techniques. Following is one of the three steps involved in optimistic concurrency control, depending on whether a transaction is a read or write operation __ For an update transaction, after validation is completed, updates held in the local variables are committed to the actual data items in the database. A) write B) read C) validate D) none of the above

Write

A transaction goes into an __ state immediately after it starts execution, where it can execute its READ and WRITE operations. A) active B) execution C) READ and WRITE D) partially committed

active state

A single SQL statement is always considered to be __ either it completes execution without an error or it fails and leaves the database unchanged. A) write B) standard C) atomic D) none of the above

atomic

When a transaction is __, it has concluded its execution successfully and all its changes must be recorded permanently in the database, even if a system failure occurs. A) committed or not B) system failure C) system log D) concurrency control

committed

When the transaction ends, it moves to the partially committed state. At this point, some types of concurrency control protocols may do additional checks to see if the transaction can be __ A) committed or not B) system failure C) system log D) concurrency control

committed or not

A single SQL statement is always considered to be atomic either it __ or it fails and leaves the database unchanged. A) completes execution without an error B) deletes redundant data C) completes then updated D) none of the above

completes execution without an error

The DBMS is responsible for ensuring that its transactions have the ACID properties. Most DBMSs include a transaction manager that coordinates the transactions associated with a database. A scheduler or a lock manager communicates with the transaction manager to implement __ A) Unrepeatable query results B) Uncommitted updates C) scheduler or a lock manager D) none of the above

concurrency control

When the transaction ends, it moves to the partially committed state. At this point, some types of __ protocols may do additional checks to see if the transaction can be committed or not A) committed or not B) system failure C) system log D) concurrency control

concurrency control protocols

One of the problems with locking methods is known as __. This occurs when two transactions are waiting for access to a data item that is locked by another transaction. A) deadlock B) read lock C) lock D) write lock

deadlock

A transaction goes into an active state immediately after it starts __, where it can execute its READ and WRITE operations. A) active state B) execution C) READ and WRITE D) partially committed

execution

a transaction can go to the failed state 1) __ or 2) if the transaction is aborted during its active state. The transaction may then have to be rolled back to undo the effect of its WRITE operations on the database. A) if the transaction is continued during its active state. B) if one of the checks fails C) if the transaction is Unrepeatable during its active state. D) if the transaction is lost during its active state.

if one of the checks fails or

a transaction can go to the 1) failed state if one of the checks fails or 2) __ The transaction may then have to be rolled back to undo the effect of its WRITE operations on the database. A) if the transaction is continued during its active state. B) if the transaction is aborted during its active state. C) if the transaction is Unrepeatable during its active state. D) if the transaction is lost during its active state.

if the transaction is aborted during its active state.

characteristics are specified by a SET TRANSACTION statement in SQL. The characteristics are the access mode, the diagnostic area size, and the isolation level. The __ option is specified using the statement which is its name, where the value can be READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, SERIALIZABLE. A) DIAGNOSTIC SIZE B) isolation level. C) SERIALIZABLE D) READ COMMITTED

isolation level.

A single SQL statement is always considered to be atomic either it completes execution without an error or __. A) completes execution and leave many error B) it fails and leaves the database unchanged C) completes then updated D) none of the above

it fails and leaves the database unchanged

A __ prevents a concurrent transaction from modifying or reading a data item while another transaction is accessing that data item A) Timestamping B) transaction C) locking and timestamp D) Concurrency control E) lock F) stamping and sealing G) optimistic

lock

Concurrency control can be viewed as a means of achieving serializability. Two main methods can be identified that allow transactions to execute concurrently and safely. These are the locking and timestamp methods. __ A lock prevents a concurrent transaction from modifying or reading a data item while another transaction is accessing that data item A) deadlock B) read lock C) lock D) write lock

lock

Concurrency control is the management of transactions that occur at the same time and is performed to ensure that the transactions do not interfere with one another. The __ and __ methods are the two main methods that allow transactions to execute concurrently and safely. A) Timestamping B) transaction C) locking and timestamp D) Concurrency control E) lock F) stamping and sealing G) optimistic

locking and timestamp

Concurrency control can be viewed as a means of achieving serializability. Two main methods can be identified that allow transactions to execute concurrently and safely. These are the A) Blocking and stopping B) locking and timestamp C) deadlocking after execution D) none of the above

locking and timestamp methods.

Concurrency control can be viewed as a means of achieving serializability. Two main methods can be identified that allow transactions to execute concurrently and safely. These are the locking and timestamp methods. Two types of locks exist: a read lock and a write lock Recall that read transactions do not conflict with one another, so __ transactions can have read locks A) one B) two C) only 3, no more no less D) none of the above

multiple transactions

The DBMS uses a serial schedule for operations that are executed consecutively without any interleaved operations occurring, and a __ for operations whose execution are interleaved with other operations. A) series schedule B) serial schedule C) program schedule D) none of the above

non-serial schedule

Every transaction has certain characteristics attributed to it. These characteristics are specified by a SET TRANSACTION statement in SQL. The characteristics are the access mode, the diagnostic area size, and the isolation level. The isolation level option is specified using the statement ISOLATION LEVEL, where the value can be READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, or SERIALIZABLE select all Possible Violations Based on Isolation Levels as Defined in SQL SERIALIZABLE A) Phantoms B) Nonrepeatable read: C) Dirty read D) none of the above

none

Concurrency control is the management of transactions that occur at the same time and is performed to ensure that the transactions do not interfere with one another. When using __ concurrency, a check is performed to determine whether a conflict is present only when a transaction is ready to be committed. If there is a conflict, the transaction is then rolled back and restarted. A) Timestamping B) transaction C) locking and timestamp D) Concurrency control E) lock F) stamping and sealing G) optimistic

optimistic

When the transaction ends, it moves to the __ state. At this point, some types of concurrency control protocols may do additional checks to see if the transaction can be committed or not A) active state B) execution C) READ and WRITE D) partially committed

partially committed

Concurrency control can be viewed as a means of achieving serializability. Two main methods can be identified that allow transactions to execute concurrently and safely. These are the locking and timestamp methods. Two types of locks exist: a read lock and a write lock. When a transaction has a __ on a data item, it can only view the item. It cannot perform any updates A) deadlock B) read lock C) lock D) write lock

read lock

a transaction can go to the failed state if one of the checks fails or if the transaction is aborted during its active state. The terminated state corresponds to the transaction leaving the system. The transaction information that is maintained in system tables while the transaction has been running is __. A) UPDATES when the transaction terminates B) removed when the transaction terminates C) Identify any relationships that have attributes. D) COMMIT when the transaction terminates E) None of the above

removed when the transaction terminates

A transaction can go to the failed state if one of the checks fails or if the transaction is aborted during its active state. Failed or aborted transactions may be __ later either automatically or after being resubmitted by the user as brand new transactions. A) ends B) restarted C) lost D) None of the above

restarted later

A transaction can go to the failed state if one of the checks fails or if the transaction is aborted during its active state. Failed or aborted transactions may be restarted later either automatically or after being ... A) updated by the user as brand new transactions. B) resubmitted by the user as brand new transactions. C) committed by the user as brand new transactions. D) none of the above

resubmitted by the user as brand new transactions.

a transaction can go to the failed state 1) if one of the checks fails or 2) if the transaction is aborted during its active state. The transaction may then have to be __ to undo the effect of its WRITE operations on the database. A) ROLLBACK B) END C) COMMIT D) UNDO

rolled back

The DBMS is responsible for ensuring that its transactions have the ACID properties. Most DBMSs include a transaction manager that coordinates the transactions associated with a database. A __ communicates with the transaction manager to implement concurrency control. A) Authorized user B) Database administrator C) scheduler or a lock manager D) none of the above

scheduler or a lock manager

select all that applies: characteristics are specified by a SET TRANSACTION statement in SQL. The characteristics are the access mode, the diagnostic area size, and the isolation level. A mode of READ WRITE allows A) select, B) update, C) modify, D) insert, E) delete, F) create commands to be executed

select, update, insert, delete, create

The DBMS uses a __ for operations that are executed consecutively without any interleaved operations occurring, and a nonserial schedule for operations whose execution are interleaved with other operations. A) series schedule B) serial schedule C) program schedule D) none of the above

serial schedule

The DBMS uses __ to find non-serial schedules that allow transactions to execute at the same time without conflict. its refers to whether a set of transactions must be executed individually, or whether they can be executed concurrently with one another it refers to whether a set of transactions must be executed individually, or whether they can be executed concurrently with one another. A) locking B) Concurrency C) serializability D) none of the above

serializability

A __ illustrates how a transaction moves through its execution states. A transaction goes into an active state immediately after it starts execution, where it can execute its READ and WRITE operations. A) national transition diagram B) transition diagram C) state transition diagram D) state shift diagram

state transition diagram

A state transition diagram illustrates how a transaction moves through its execution states. some recovery protocols need to ensure that a __ will not result in an inability to record the changes of the transaction permanently (usually by recording changes in the system log). A) committed or not B) system failure C) system log D) concurrency control

system failure

A state transition diagram illustrates how a transaction moves through its execution states. some recovery protocols need to ensure that a system failure will not result in an inability to record the changes of the transaction permanently (usually by recording changes in the __). A) committed or not B) system failure C) system log D) concurrency control

system log

DBMSs include methods to detect and prevent deadlocks. One popular method is __ The basic idea behind this is to order all transactions so that the oldest transaction receives priority in case of a data access conflict. No locks are involved, so there can be no deadlocks. A) deadlock B) read lock C) lock D) timestamps

timestamps

DBMSs include methods to detect and prevent deadlocks. One popular method is __. This provides an alternative approach to realizing serializability. this a distinct identifier that the DBMS assigns to a transaction, specifies the relative start time of the transaction. A) deadlock B) read lock C) lock D) timestamps

timestamps

A __is an operation, or series of operations, conducted by a single user against a database that retrieves or alters the data in the database. A) Timestamping B) transaction C) locking and timestamp D) Concurrency control E) lock F) stamping and sealing G) optimistic

transaction

A __is an operation, or series of operations, conducted by a single user against a database that retrieves or alters the data in the database. It be the result of an entire or partial program, or it can be as simple as a single SQL statement executed by a user. Also it is not limited to one type of operation against the database but can include various types A) rollback B) commit C) transaction D) aborted

transaction

The DBMS is responsible for ensuring that its transactions have the ACID properties. Most DBMSs include a __ that coordinates the transactions associated with a database. A) Database administrator B) transaction manager C) enterprise rule and regulations D) none of the above

transaction manager

Concurrency control can be viewed as a means of achieving serializability. Two main methods can be identified that allow transactions to execute concurrently and safely. These are the locking and timestamp methods. Two types of locks exist: a read lock and a write lock. Conversely, if a transaction has a __ on a data item, no other transactions can read or write to that data item A) deadlock B) read lock C) lock D) write lock

write lock

Concurrency control can be viewed as a means of achieving serializability. Two main methods can be identified that allow transactions to execute concurrently and safely. These are the locking and timestamp methods. Two types of locks exist: a read lock and a write lock. When a transaction has a __ on a data item, it can read and write to the data item. A) deadlock B) read lock C) lock D) write lock

write lock