Join processing in relational databases

Proc. of the Int'l Conference on Information …

Most join algorithms can be extended to reduce wasted work when several tuples contain the same value of the join attribute. We show that separating detection of duplicates from their exploitation improves modularity and makes it easier to implement whole families of hierarchy-exploiting join algorithms that avoid duplication. The technique is also used to provide an execution technique for star-like patterns of joins around a central relation. It dominates Ingres-like substitution for the central relation, in both performance and ease of including in a conventional optimizer. Its performance dominates a cascade of conventional binary joins, and performance estimates are more accurate. We then argue that such techniques make it undesirable to implement physical-level multiway join operations within a query processor.

Information Systems, 1992

When SQL is used to formulate queries for a relational database, many conditions in the WHERE clause appear to be very predictable. These are the so-called join conditions which indicate how the tables of a database are related. It seems that a system should be able to generate these conditions to a great extent automatically from the knowledge of the database structure. To this end the notion of the structure of a join is introduced and mathematically described as a graph morphism. It turns out to be a generalization of the notion of a natural join. It is claimed that this approach is theoretically elegant and provides in practice a good basis for the development of query generators.

Join is an operation in accessing the data from table if number of tables exceeds one. Whenever we need the data which is not available from a single table, then it needs to necessitate using join operation. Sometimes join is required even if there is a single table. It all depends on the format in which we need to display the data in the user environment.

1990

In relational databases, all relations are at least in First-Normal-Form (1NF) which requires all attributes to have atomic domains. That is, elements of the domains are considered to be indivisible units. In the nested relational model – as an extension of the relational model – domains may be either atomic or relation valued. That is, an attribute value of a tuple can be a relation. title author-list date keyword-list DB Theory {Smith, Jones} 1 April 79 {algebra, logic} Programming {Jones, Frick} 17 June 85 {Pascal, C} A nested relation can be decomposed (" flattened ") into a relation having the 1NF property. It then can be decomposed into a set of relations that satisfy the Third Normal Form (3NF). Nested relations are based on a type constructor for collection types. Requires operators to " flatten " nested relation. Another object-relational feature provided in Oracle 10g is the ability to have a nested relation. • The keyword table allows you to treat a ne...

2002

Although communication cost is still a major cost for distributed databases, local cost in distributed q u e y processing cannot be neglecte~1JfzJf3JfsJf10J,. Observing the fact that almost all commercial database products employ Plan Enumeration with Dynamic Programming (PEDP) techniques lZJ, we find reducing the cost of both communication and local processing in 2-way join has potential benejits. Although many methods for reducing communication cost have been proposed, most of them employ a cost model that neglects local processing cost. This paper proposes a join execution method (called virtual join) that considers both of them. Virtual join has two desirable features: I) Being adaptive to different values of selectivity. 2) Giving accurate cardinality of join result before it is materialized. Experiment results showed virtual join was both adaptive and efficient.

This article presents an approach of the cost model used in join optimization. The search space is determined through transformations on the query blocks, depending on the selection predicate. Different implementations of the JOIN operator are taken into account for cost evaluation of the execution plans.

1977

We define the class of conjunctive queries in relational data bases, and the generalized join operator on relations.

9th International Database Engineering & Application Symposium (IDEAS'05), 2005

We introduce and study a new class of queries that we refer to as ACMA (arithmetic constraints on multiple attributes) queries. Such combinatorial queries require the simultaneous satisfaction of arithmetic constraints on three or more attributes from multiple relations, and thus often involve expensive combinatorial search. Building on techniques from constraint programming, we develop algorithms, preprocessing methods, index structures, and a new constrained join operator that allow ACMA queries to be evaluated efficiently within a conventional relational database engine. We present the results of a careful performance evaluation of both our new approach and the conventional nested-loop join algorithm. Measurements of tuples read, intermediate results generated, and execution time show that our approach achieves superior performance for ACMA joins. By thus showing how a database system can be extended with constraint solving algorithms to perform efficient joins in the presence of ACMAs, we extend the range of applicability of relational databases into an important new area.

Finding the optimal join ordering for a database query is a complex combinatorial optimization problem which has been approached by a wide variety of strategies and algorithms, ranging from simple deterministic search to complex hybrid algorithms based on genetic search and incorporating domain-specific heuristics. In this paper we review a set of join ordering algorithms and classify them according to the nature of the search strategy they implement. We also briefly discuss the relative advantages and applicability of different algorithms.