The DDL Class Declaration

A class declaration defines a group of related class-elements (or fields) that are stored together. A class declaration consists of class element statements, zero or more access statements, and zero or more event statements with syntax as follows:

    [compact] [persistent | transient] class class-name { class-elements-list };

where a class-elements-list consists of:

    element-statement | access-statement | event-statement;
    [element-statement | access-statement | event-statement;]
    […;]

The element-statement has syntax as follows according to the type of field being declared:

    [nullable] type-identifier | struct-name | enum element-name[length] [ = value [, element-name [length] [= value]] …];

or

    vector { type-identifier | struct-name } vector-name;

or

    [optional] struct-name element-name;

The access-statement has syntax as follows according to the type of index being declared:

    [voluntary] [unique] [inclusive] [userdef] tree < class-element | struct-name.element-name | vector-element
        [asc|desc] [collate collation_name] [, class-element | struct-name.element-name | vector-element [asc|desc]
        [collate collation_name] …] >
    index-name;

or

    [userdef] [unique|nonunique] hash < class-element | struct-name.element-name | vector-element
        [collate collation_name] [, class-element | struct-name.element-name | vector-element
        [collate collation_name] …] >
    index-name[expected-number-of-entries];

or

    trie < class-element [, class-element] > index-name;

or

    trigram < class-element [, class-element] > index-name;

or

    [unique] rtree < class-element > index-name;

or

    kdtree < class-element [, class-element] > index-name;

or

    hnsw < class-element [, class-element] > index-name;

or

    oid;

or

    autoid[number-of-expected-entries];

or

    list;

The event-statement has syntax as follows according to the type of event being declared:

    event < [[class-element] update | new | delete | delete_all | checkpoint] > event_name [; event-statement];

where class-element has syntax:

    element-name | struct-name

and vector-element has syntax:

    vector-name.struct-element | vector-name;

The Compact Qualifier

The compact class qualifier limits the total size of the class' elements to 64K. (That includes not just application data, but also the overhead required by the runtime.)

Note: The compact declaration significantly reduces the overhead that is necessary to maintain the class data (simply put, all references within a class are 2 bytes instead of 4 bytes). It is easy to estimate the total number of references within an object—each optional, vector, string, or blob field takes one reference.