Query evaluation
determining the answers to a query on a database
In database theory, the query evaluation problem is the problem of determining the answers to a query on a database. Research in database theory aims at determining the computational complexity of answering different kinds of queries over databases, in particular over relational databases.
Formal definition
The query evaluation problem takes two inputs: the query to be answered, and the database on which to answer it. The output of the problem is the set of answers to the query on the database. If the queries are Boolean queries, i.e., queries have a yes or no answer (for example, Boolean conjunctive queries) then the query evaluation problem is a decision problem.
The query evaluation problem is usually posed for a specific class of queries and databases. For instance, one example of the query evaluation problem would be the problem of evaluating a conjunctive query on a relational database.
The computational complexity of the problem can be measured in different ways, to account for the fact that the two inputs of the problem are different:
The combined complexity of the query evaluation problem is its computational complexity when measured as a function of the two inputs, i.e., the query and the database, as usual in computational complexity.
The data complexity is the computational complexity when the query is fixed and when the input is just the database. For instance, we say that query evaluation has polynomial-time data complexity for a class of queries if, for every fixed query Q in that class, given a database D, we can compute the answers to Q on D in polynomial time.
Begin with the source’s own compact description: “Query evaluation” is determining the answers to a query on a database. The dossier treats that line as a proposition to test through Query, evaluation and determining, not as a finished interpretation.
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Named sources, stable identifiers and responsible institutions provide the strongest route from overview to verifiable evidence. The source revision retrieved here is dated Feb 4, 2026. The linked authority identifier is Q124304308. None of the 0 selected statements returned an explicit reference.
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This entry incorporates text from “Query evaluation” on English Wikipedia. Contributors are listed in the page history. Text is available under the Creative Commons Attribution-ShareAlike 4.0 License. Selected authority identifiers and statements are retrieved from Wikidata under CC0; their references and qualifiers remain part of the verification path.