diff --git a/documentation/src/main/asciidoc/introduction/Advanced.adoc b/documentation/src/main/asciidoc/introduction/Advanced.adoc index f84e74ead9..5bf1303cc3 100644 --- a/documentation/src/main/asciidoc/introduction/Advanced.adoc +++ b/documentation/src/main/asciidoc/introduction/Advanced.adoc @@ -640,6 +640,62 @@ from Order ord Polymorphic association joins for `@Any` mappings are not currently implemented. ==== +[[dynamic-insert-update]] +=== Selective columns inserts and updates + +By default, Hibernate generates `insert` and `update` statements for each entity during boostrap, and reuses the same `insert` statement every time an instance of the entity is made persistent, and the same `update` statement every time an instance of the entity is modified. + +This means that: + +- if an attribute is `null` when the entity is made persistent, its mapped column is redundantly included in the SQL `insert`, and +- worse, if a certain attribute is unmodified when other attributes are changed, the column mapped by that attribute is redundantly included in the SQL `update`. + +Most of the time this just isn't an issue worth worrying about. +The cost of interacting with the database is _usually_ dominated by the cost of a round trip, not by the number of columns in the `insert` or `update`. +But in cases where it does become important, there are two ways to be more selective about which columns are included in the SQL. + +The JPA-standard way is to indicate statically which columns are eligible for inclusion via the `@Column` annotation. +For example, if an entity is always created with an immutable `creationDate`, and with no `completionDate`, then we would write: + +[source,java] +---- +@Column(updatable=false) LocalDate creationDate; +@Column(insertable=false) LocalDate completionDate; +---- + +This approach works quite well in many cases, but often breaks down for entities with more than a handful of updatable columns. + +An alternative solution is to ask Hibernate to generate SQL dynamically each time an `insert` or `update` is executed. +We do this by annotating the entity class. + +.Annotations for dynamic SQL generation +[%breakable,cols="25,~"] +|=== +| Annotation | Purpose + +| `@DynamicInsert` | Specifies that an `insert` statement should be generated each time an entity is made persistent +| `@DynamicUpdate` | Specifies that an `update` statement should be generated each time an entity is modified +|=== + +It's important to realize that, while `@DynamicInsert` has no impact on semantics, the more useful `@DynamicUpdate` annotation _does_ have a subtle side effect. + +[CAUTION] +==== +The wrinkle is that if an entity has no version property, `@DynamicUpdate` opens the possibility of two optimistic transactions concurrently reading and selectively updating a given instance of the entity. +In principle, this might lead to a row with inconsistent column values after both optimistic transactions commit successfully. +==== + +Of course, this consideration doesn't arise for entities with a `@Version` attribute. + +[TIP] +==== +But there's a solution! +Well-designed relational schemas should have _constraints_ to ensure data integrity. +That's true no matter what measures we take to preserve integrity in our program logic. +We may ask Hibernate to add a `check` constraint to our table using the `@Check` annotation. +Check constraints and foreign key constraints can help ensure that a row never contains inconsistent column values. +==== + [[bytecode-enhancer]] === Using the bytecode enhancer