56 lines
2.2 KiB
Plaintext
56 lines
2.2 KiB
Plaintext
= SQM
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The Semantic Query Model (SQM) is Hibernate's representation of an HQL or Criteria query's semantic (meaning). This
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representation is modeled as an "abstract syntax tree" (AST) - meaning it is a structured tree of nodes where each node
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represents an atomic piece of the query. E.g. `SqmSelectClause` represents the query's select clause.
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`SqmSelectClause` is ultimately a collection of one or more `SqmSelection` references representing the individual
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selections to be returned from the query (called the domain results). Etc
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All of these details are handled by the `QuerySqmImpl` implementation of `Query`. This is what Hibernate
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uses for both HQL and Criteria queries.
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== The Tree
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SQM is an Abstract Syntax Tree (AST) describing a query from the perspective of the domain model. It uses the Hibernate
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domain model, which is Hibernate's extension to the JPA model. This model contains no relational mapping information -
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it simply describes the domain model in mostly Java terms although it does incorporate "classifications" of the type
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system. E.g. it understand that `Customer` is an entity, but contains no information about the tables, columns, etc.
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See the `design/type-system-domain.adoc` design doc. For details about this domain model
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The tree model is defined in the package `org.hibernate.query.sqm.tree`
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== Building an SQM - HQL
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`org.hibernate.query.hql.HqlTranslator#translate`
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== Building an SQM - Criteria
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`org.hibernate.query.sqm.internal.SqmCriteriaNodeBuilder`
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== Translating an SQM
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Generic support for walking over the SQM tree via the `org.hibernate.query.sqm.SemanticQueryWalker` contract.
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More specialized support specifically for translating the SQM into a SQL AST is also defined by
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`org.hibernate.query.sqm.sql.SqmToSqlAstConverter`.
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[source]
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----
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final SessionFactoryImplementor sessionFactory = ...;
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final QueryEngine queryEngine = sessionFactory.getQueryEngine();
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final SqmTranslatorFactory sqmTranslatorFactory = queryEngine.getSqmTranslatorFactory();
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final SqmSelectTranslator sqmConverter = sqmTranslatorFactory.createSelectTranslator( ... );
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final SqmSelectTranslation interpretation = sqmConverter.translate( sqm );
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----
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See the document `design/sql-ast.adoc` for details about the SQL AST, including its execution.
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