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...nificantly lowered whenever it is necessary to leave some congested node locked (a congested node is one that is often accessed, e.g., the root of a tree) while waiting for a secondary memory access. =-=(4)-=- To allow a transaction to abort itself when mistakes occur, locks cannot be released until the end of the transaction. This may again significantly lower concurrency. (5) Most important for the purpo...

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...r a secondary memory access. (4) To allow a transaction to abort itself when mistakes occur, locks cannot be released until the end of the transaction. This may again significantly lower concurrency. =-=(5)-=- Most important for the purposes of this paper, locking may be necessary only in the worst case. Consider the following simple example: The directed graph consists solely of roots, and each transactio...

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...he data is called a transaction (see, e.g., [4]). If our goal is to maximize the throughput of accesses to the database, then there are at least two cases where highly concurrent access is desirable. =-=(1)-=- The amount of data is sufficiently great that at any given time only a fraction of the database can be present in primary memory, so that it is necessary to swap parts of the database from secondary ...

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...a is sufficiently great that at any given time only a fraction of the database can be present in primary memory, so that it is necessary to swap parts of the database from secondary memory as needed. =-=(2)-=- Even if the entire database can be present in primary memory, there may be multiple processors. In both cases the hardware will be underutilized if the degree of concurrency is too low. However, as i...

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... been directed at developing special-purpose locking protocols for various special cases. For example, in the case of B-trees [l], at least nine locking protocols have been proposed [2, 3,9, 10, 131. =-=(3)-=- In the case that large parts of the database are on secondary memory, concurrency is significantly lowered whenever it is necessary to leave some congested node locked (a congested node is one that i...

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... PHASE A widely used criterion for verifying the correctness of concurrent execution of transactions has been variously called serial equivalence [4], serial reproducibility [ll], and linearizability =-=[14]-=-. This criterion may be defined as follows. Let transactions Tl, TX, . . . ,T, be executed concurrently. Denote an instance of the shared data structure by d, and let D be the set of all possible d, s...

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...d (b) every concurrent execution of transaction is serially equivalent than it is to verify directly that every concurrent execution of transactions preserves integrity. In fact, it has been shown in =-=[7]-=- that serialization is the weakest criterion for preserving consistency of a concurrent transaction system, even if complete syntactic information of the system is available to the concurrency control...

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...before Tj starts writing; it simply requires that Ti not affect the read phase or the write phase of Tj (again note that T; cannot affect the read phase of Ti, by the last part of the condition). See =-=[12]-=- for a set of similar conditions for serialization. ACM Transactions on Database Systems, Vol. 6, No. 2, June 1981.srj)--f--t---l (2) r;.t--+-H 3.2 Assigning Transaction Numbers Optimistic Methods for...