Turn complex GraphQL queries into optimized database queries.

pip install graphql-compiler

For a more detailed overview and getting started guide, please see our blog post.

To pretty-print GraphQL queries, use the included pretty-printer:

python -m graphql_compiler.tool <input_file.graphql >output_file.graphql

It's modeled after Python's json.tool, reading from stdin and writing to stdout.

• FAQ
• Definitions
• Directives
  • @optional
  • @output
  • @fold
  • @tag
  • @filter
  • @recurse
  • @output_source
• Supported filtering operations
  • Comparison operators
  • name_or_alias
  • between
  • in_collection
  • has_substring
  • contains
  • has_edge_degree
• Type coercions
• Meta fields
• The GraphQL schema
• Execution model
• Miscellaneous
  • Expanding @optional vertex fields
  • Optional type_equivalence_hints compilation parameter
• License

Q: Does the compiler support all GraphQL language features?

A: No. We prioritized and implemented a subset of all functionality supported by the GraphQL language. We hope to add more functionality over time.

Q: Do you really use GraphQL, or do you just use GraphQL-like syntax?

A: We really use GraphQL. Any query that the compiler will accept is entirely valid GraphQL, and we actually use the Python port of the GraphQL core library for parsing and type checking. However, since the database queries produced by compiling GraphQL are subject to the limitations of the database system they run on, our execution model is somewhat different compared to the one described in the standard GraphQL specification. See the Execution model section for more details.

Q: Does this project come with a GraphQL server implementation?

A: No -- there are many existing frameworks for running a web server. We simply built a tool that takes GraphQL query strings (and their parameters) and returns a query string you can use with your database. The compiler does not execute the query string against the database, nor does it deserialize the results. Therefore, it is agnostic to the choice of server framework and database client library used.

Q: What databases and query languages does the compiler support?

A: We currently support a single database, OrientDB version 2.2.28+, and two query languages that OrientDB supports: the OrientDB dialect of gremlin, and OrientDB's own custom SQL-like query language that we refer to as MATCH, after the name of its graph traversal operator. With OrientDB, MATCH should be the preferred choice for most users, since it tends to run faster than gremlin, and has other desirable properties. See the Execution model section for more details.

Q: Do you plan to support other databases / more GraphQL features in the future?

A: We'd love to, and we could really use your help! Please consider contributing to this project by opening issues, opening pull requests, or participating in discussions.

Q: I think I found a bug, what do I do?

A: Please check if an issue has already been created for the bug, and open a new one if not. Make sure to describe the bug in as much detail as possible, including any stack traces or error messages you may have seen, which database you're using, and what query you compiled.

Q: I think I found a security vulnerability, what do I do?

A: Please reach out to us at graphql-compiler-maintainer@kensho.com so we can triage the issue and take appropriate action.

• Vertex field: A field corresponding to a vertex in the graph. In the below example, Animal and out_Entity_Related are vertex fields. The Animal field is the field at which querying starts, and is therefore the root vertex field. In any scope, fields with the prefix out_ denote vertex fields connected by an outbound edge, whereas ones with the prefix in_ denote vertex fields connected by an inbound edge.

{
    Animal {
        name @output(out_name: "name")
        out_Entity_Related {
            ... on Species {
                description @output(out_name: "description")
            }
        }
    }
}

• Property field: A field corresponding to a property of a vertex in the graph. In the above example, the name and description fields are property fields. In any given scope, property fields must appear before vertex fields.
• Result set: An assignment of vertices in the graph to scopes (locations) in the query. As the database processes the query, new result sets may be created (e.g. when traversing edges), and result sets may be discarded when they do not satisfy filters or type coercions. After all parts of the query are processed by the database, all remaining result sets are used to form the query result, by taking their values at all properties marked for output.
• Scope: The part of a query between any pair of curly braces. The compiler infers the type of each scope. For example, in the above query, the scope beginning with Animal { is of type Animal, the one beginning with out_Entity_Related { is of type Entity, and the one beginning with ... on Species { is of type Species.
• Type coercion: An operation that produces a new scope of narrower type than the scope in which it exists. Any result sets that cannot satisfy the narrower type are filtered out and not returned. In the above query, ... on Species is a type coercion which takes its enclosing scope of type Entity, and coerces it into a narrower scope of type Species. This is possible since Entity is an interface, and Species is a type that implements the Entity interface.

Without this directive, when a query includes a vertex field, any results matching that query must be able to produce a value for that vertex field. Applied to a vertex field, this directive prevents result sets that are unable to produce a value for that field from being discarded, and allowed to continue processing the remainder of the query.

{
    Animal {
        name @output(out_name: "name")
        out_Animal_ParentOf @optional {
            name @output(out_name: "child_name")
        }
    }
}

For each Animal:

• if it is a parent of another animal, at least one row containing the parent and child animal's names, in the name and child_name columns respectively;
• if it is not a parent of another animal, a row with its name in the name column, and a null value in the child_name column.

• @optional can only be applied to vertex fields, except the root vertex field.
• It is allowed to expand vertex fields within an @optional scope. However, doing so is currently associated with a performance penalty in MATCH. For more detail, see: Expanding @optional vertex fields.
• @recurse, @fold, or @output_source may not be used at the same vertex field as @optional.
• @optional, @output_source and @fold may not be used anywhere within a scope marked @optional.

If a given result set is unable to produce a value for a vertex field marked @optional, any fields marked @output within that vertex field return the null value.

When filtering (via @filter) or type coercion (via e.g. ... on Animal) are applied at or within a vertex field marked @optional, the @optional is given precedence:

• If a given result set cannot produce a value for the optional vertex field, it is preserved: the @optional directive is applied first, and no filtering or type coercion can happen.
• If a given result set is able to produce a value for the optional vertex field, the @optional does not apply, and that value is then checked against the filtering or type coercion. These subsequent operations may then cause the result set to be discarded if it does not match.

Denotes that the value of a property field should be included in the output. Its out_name argument specifies the name of the column in which the output value should be returned.

{
    Animal {
        name @output(out_name: "animal_name")
    }
}

This query returns the name of each Animal in the graph, in a column named animal_name.

• @output can only be applied to property fields.
• The value provided for out_name may only consist of upper or lower case letters (A-Z, a-z), or underscores (_).
• For any given query, all out_name values must be unique. In other words, output columns must have unique names.

If the property field marked @output exists within a scope marked @optional, result sets that are unable to assign a value to the optional scope return the value null as the output of that property field.

Applying @fold on a scope "folds" all outputs from within that scope: rather than appearing on separate rows in the query result, the folded outputs are coalesced into lists starting at the scope marked @fold.

{
    Animal {
        name @output(out_name: "animal_name")
        out_Animal_ParentOf @fold {
            name @output(out_name: "child_names")
        }
    }
}

Each returned row has two columns: animal_name with the name of each Animal in the graph, and child_names with a list of the names of all children of the Animal named animal_name. If a given Animal has no children, its child_names list is empty.

• @fold can only be applied to vertex fields, except the root vertex field.
• May not exist at the same vertex field as @recurse, @optional, or @output_source.
• Any scope that is either marked with @fold or is nested within a @fold marked scope, may expand at most one vertex field.
• There must be at least one @output field within a @fold scope.
• All @output fields within a @fold traversal must be present at the innermost scope. It is invalid to expand vertex fields within a @fold after encountering an @output directive.
• @tag, @recurse, @optional, @output_source and @fold may not be used anywhere within a scope marked @fold.
• Use of type coercions or @filter at or within the vertex field marked @fold is allowed. Only data that satisfies the given type coercions and filters is returned by the @fold.
• If the compiler is able to prove that the type coercion in the @fold scope is actually a no-op, it may optimize it away. See the Optional type_equivalence_hints compilation parameter section for more details.

The following GraphQL is not allowed and will produce a GraphQLCompilationError. This query is invalid for two separate reasons:

• It expands vertex fields after an @output directive (outputting animal_name)
• The in_Animal_ParentOf scope, which is within a scope marked @fold, expands two vertex fields instead of at most one.

{
    Animal {
        out_Animal_ParentOf @fold {
            name @output(out_name: "animal_name")
            in_Animal_ParentOf {
                out_Animal_OfSpecies {
                    uuid @output(out_name: "species_id")
                }
                out_Animal_RelatedTo {
                    name @output(out_name: "relative_name")
                }
            }
        }
    }
}

The following is a valid use of @fold:

{
    Animal {
        out_Animal_ParentOf @fold {
            in_Animal_ParentOf {
                in_Animal_ParentOf {
                    out_Animal_RelatedTo {
                        name @output(out_name: "final_name")
                    }
                }
            }
        }
    }
}

The @tag directive enables filtering based on values encountered elsewhere in the same query. Applied on a property field, it assigns a name to the value of that property field, allowing that value to then be used as part of a @filter directive.

To supply a tagged value to a @filter directive, place the tag name (prefixed with a % symbol) in the @filter's value array. See Passing parameters for more details.

{
    Animal {
        name @tag(tag_name: "parent_name")
        out_Animal_ParentOf {
            name @filter(op_name: "<", value: ["%parent_name"])
                 @output(out_name: "child_name")
        }
    }
}

Each row returned by this query contains, in the child_name column, the name of an Animal that is the child of another Animal, and has a name that is lexicographically smaller than the name of its parent.

• @tag can only be applied to property fields.
• The value provided for tag_name may only consist of upper or lower case letters (A-Z, a-z), or underscores (_).
• For any given query, all tag_name values must be unique.
• Cannot be applied to property fields within a scope marked @fold.

Allows filtering of the data to be returned, based on any of a set of filtering operations. Conceptually, it is the GraphQL equivalent of the SQL WHERE keyword.

See Supported filtering operations for details on the various types of filtering that the compiler currently supports. These operations are currently hardcoded in the compiler; in the future, we may enable the addition of custom filtering operations via compiler plugins.

Multiple @filter directives may be applied to the same field at once. Conceptually, it is as if the different @filter directives were joined by SQL AND keywords.

The @filter directive accepts two types of parameters: runtime parameters and tagged parameters.

Runtime parameters are represented with a $ prefix (e.g. $foo), and denote parameters whose values will be known at runtime. The compiler will compile the GraphQL query leaving a spot for the value to fill at runtime. After compilation, the user will have to supply values for all runtime parameters, and their values will be inserted into the final query before it can be executed against the database.

Consider the following query:

{
    Animal {
        name @output(out_name: "animal_name")
        color @filter(op_name: "=", value: ["$animal_color"])
    }
}

It returns one row for every Animal that has a color equal to $animal_color, containing the animal's name in a column named animal_name. The parameter $animal_color is a runtime parameter -- the user must pass in a value (e.g. {"$animal_color": "blue"}) that will be inserted into the query before querying the database.

Tagged parameters are represented with a % prefix (e.g. %foo) and denote parameters whose values are derived from a property field encountered elsewhere in the query. If the user marks a property field with a @tag directive and a suitable name, that value becomes available to use as a tagged parameter in all subsequent @filter directives.

Consider the following query:

{
    Animal {
        name @tag(out_name: "parent_name")
        out_Animal_ParentOf {
            name @filter(op_name: "has_substring", value: ["%parent_name"])
                 @output(out_name: "child_name")
        }
    }
}

It returns the names of animals that contain their parent's name as a substring of their own. The database captures the value of the parent animal's name as the parent_name tag, and this value is then used as the %parent_name tagged parameter in the child animal's @filter.

We considered and rejected the idea of allowing literal values (e.g. 123) as @filter parameters, for several reasons:

• The GraphQL type of the @filter directive's value field cannot reasonably encompass all the different types of arguments that people might supply. Even counting scalar types only, there's already ID, Int, Float, Boolean, String, Date, DateTime... -- way too many to include.
• Literal values would be used when the parameter's value is known to be fixed. We can just as easily accomplish the same thing by using a runtime parameter with a fixed value. That approach has the added benefit of potentially reducing the number of different queries that have to be compiled: two queries with different literal values would have to be compiled twice, whereas using two different sets of runtime arguments only requires the compilation of one query.
• We were concerned about the potential for accidental misuse of literal values. SQL systems have supported stored procedures and parameterized queries for decades, and yet ad-hoc SQL query construction via simple string interpolation is still a serious problem and is the source of many SQL injection vulnerabilities. We felt that disallowing literal values in the query will drastically reduce both the use and the risks of unsafe string interpolation, at an acceptable cost.

• The value provided for op_name may only consist of upper or lower case letters (A-Z, a-z), or underscores (_).
• Values provided in the value list must start with either $ (denoting a runtime parameter) or % (denoting a tagged parameter), followed by exclusively upper or lower case letters (A-Z, a-z) or underscores (_).
• The @tag directives corresponding to any tagged parameters in a given @filter query must be applied to fields that appear strictly before the field with the @filter directive.
• "Can't compare apples and oranges" -- the GraphQL type of the parameters supplied to the @filter must match the GraphQL types the compiler infers based on the field the @filter is applied to.
• If the @tag corresponding to a tagged parameter originates from within a vertex field marked @optional, the emitted code for the @filter checks if the @optional field was assigned a value. If no value was assigned to the @optional field, comparisons against the tagged parameter from within that field return True.
  • For example, assuming %from_optional originates from an @optional scope, when no value is assigned to the @optional field:
    • using @filter(op_name: "=", value: ["%from_optional"]) is equivalent to not having the filter at all;
    • using @filter(op_name: "between", value: ["$lower", "%from_optional"]) is equivalent to @filter(op_name: ">=", value: ["$lower"]).

Applied to a vertex field, specifies that the edge connecting that vertex field to the current vertex should be visited repeatedly, up to depth times. The recursion always starts at depth = 0, i.e. the current vertex -- see the below sections for a more thorough explanation.

Say the user wants to fetch the names of the children and grandchildren of each Animal. That could be accomplished by running the following two queries and concatenating their results:

{
    Animal {
        name @output(out_name: "ancestor")
        out_Animal_ParentOf {
            name @output(out_name: "descendant")
        }
    }
}

{
    Animal {
        name @output(out_name: "ancestor")
        out_Animal_ParentOf {
            out_Animal_ParentOf {
                name @output(out_name: "descendant")
            }
        }
    }
}

If the user then wanted to also add great-grandchildren to the descendants output, that would require yet another query, and so on. Instead of concatenating the results of multiple queries, the user can simply use the @recurse directive. The following query returns the child and grandchild descendants:

{
    Animal {
        name @output(out_name: "ancestor")
        out_Animal_ParentOf {
            out_Animal_ParentOf @recurse(depth: 1) {
                name @output(out_name: "descendant")
            }
        }
    }
}

Each row returned by this query contains the name of an Animal in the ancestor column and the name of its child or grandchild in the descendant column. The out_Animal_ParentOf vertex field marked @recurse is already enclosed within another out_Animal_ParentOf vertex field, so the recursion starts at the "child" level (the out_Animal_ParentOf not marked with @recurse). Therefore, the descendant column contains the names of an ancestor's children (from depth = 0 of the recursion) and the names of its grandchildren (from depth = 1).

Recursion using this directive is possible since the types of the enclosing scope and the recursion scope work out: the @recurse directive is applied to a vertex field of type Animal and its vertex field is enclosed within a scope of type Animal. Additional cases where recursion is allowed are described in detail below.

The descendant column cannot have the name of the ancestor animal since the @recurse is already within one out_Animal_ParentOf and not at the root Animal vertex field. Similarly, it cannot have descendants that are more than two steps removed (e.g., great-grandchildren), since the depth parameter of @recurse is set to 1.

Now, let's see what happens when we eliminate the outer out_Animal_ParentOf vertex field and simply have the @recurse applied on the out_Animal_ParentOf in the root vertex field scope:

{
    Animal {
        name @output(out_name: "ancestor")
        out_Animal_ParentOf @recurse(depth: 1) {
            name @output(out_name: "self_or_descendant")
        }
    }
}

In this case, when the recursion starts at depth = 0, the Animal within the recursion scope will be the same Animal at the root vertex field, and therefore, in the depth = 0 step of the recursion, the value of the self_or_descendant field will be equal to the value of the ancestor field.

• "The types must work out" -- when applied within a scope of type A, to a vertex field of type B, at least one of the following must be true:
  • A is a GraphQL union;
  • B is a GraphQL interface, and A is a type that implements that interface;
  • A and B are the same type.
• @recurse can only be applied to vertex fields other than the root vertex field of a query.
• Cannot be used within a scope marked @optional or @fold.
• The depth parameter of the recursion must always have a value greater than or equal to 1. Using depth = 1 produces the current vertex and its neighboring vertices along the specified edge.
• Type coercions and @filter directives within a scope marked @recurse do not limit the recursion depth. Conceptually, recursion to the specified depth happens first, and then type coercions and @filter directives eliminate some of the locations reached by the recursion.
• As demonstrated by the examples above, the recursion always starts at depth 0, so the recursion scope always includes the vertex at the scope that encloses the vertex field marked @recurse.

See the Completeness of returned results section for a description of the directive and examples.

• May exist at most once in any given GraphQL query.
• Can exist only on a vertex field, and only on the last vertex field used in the query.
• Cannot be used within a scope marked @optional or @fold.

Supported comparison operators:

• Equal to: =
• Not equal to: !=
• Greater than: >
• Less than: <
• Greater than or equal to: >=
• Less than or equal to: <=

{
    Species {
        name @filter(op_name: "=", value: ["$species_name"])
        uuid @output(out_name: "species_uuid")
    }
}

This returns one row for every Species whose name is equal to the value of the $species_name parameter, containing the uuid of the Species in a column named species_uuid.

{
    Animal {
        name @output(out_name: "name")
        birthday @output(out_name: "birthday")
                 @filter(op_name: ">=", value: ["$point_in_time"])
    }
}

This returns one row for every Animal that was born after or on a $point_in_time, containing the animal's name and birthday in columns named name and birthday, respectively.

• All comparison operators must be on a property field.

Allows you to filter on vertices which contain the exact string $wanted_name_or_alias in their name or alias fields.

{
    Animal @filter(op_name: "name_or_alias", value: ["$wanted_name_or_alias"]) {
        name @output(out_name: "name")
    }
}

This returns one row for every Animal whose name and/or alias is equal to $wanted_name_or_alias, containing the animal's name in a column named name.

The value provided for $wanted_name_or_alias must be the full name and/or alias of the Animal. Substrings will not be matched.

• Must be on a vertex field that has name and alias properties.

{
    Animal {
        name @output(out_name: "name")
        birthday @filter(op_name: "between", value: ["$lower", "$upper"])
                 @output(out_name: "birthday")
    }
}

This returns:

• One row for every Animal whose birthday is in between $lower and $upper dates (inclusive), containing the animal's name in a column named name.

• Must be on a property field.
• The lower and upper bounds represent an inclusive interval, which means that the output may contain values that match them exactly.

{
    Animal {
        name @output(out_name: "animal_name")
        color @output(out_name: "color")
              @filter(op_name: "in_collection", value: ["$colors"])
    }
}

This returns one row for every Animal which has a color contained in a list of colors, containing the Animal's name and color in columns named animal_name and color, respectively.

• Must be on a property field that is not of list type.

{
    Animal {
        name @filter(op_name: "has_substring", value: ["$substring"])
             @output(out_name: "animal_name")
    }
}

This returns one row for every Animal whose name contains the value supplied for the $substring parameter. Each row contains the matching Animal's name in a column named animal_name.

• Must be on a property field of string type.

{
    Animal {
        alias @filter(op_name: "contains", value: ["$wanted"])
        name @output(out_name: "animal_name")
    }
}

This returns one row for every Animal whose list of aliases contains the value supplied for the $wanted parameter. Each row contains the matching Animal's name in a column named animal_name.

• Must be on a property field of list type.

{
    Animal {
        name @output(out_name: "animal_name")

        out_Animal_ParentOf @filter(op_name: "has_edge_degree", value: ["$child_count"]) @optional {
            uuid
        }
    }
}

This returns one row for every Animal that has exactly $child_count children (i.e. where the out_Animal_ParentOf edge appears exactly $child_count times). Each row contains the matching Animal's name, in a column named animal_name.

The uuid field within the out_Animal_ParentOf vertex field is added simply to satisfy the GraphQL syntax rule that requires at least one field to exist within any {}. Since this field is not marked with any directive, it has no effect on the query.

N.B.: Please note the @optional directive on the vertex field being filtered above. If in your use case you expect to set $child_count to 0, you must also mark that vertex field @optional. Recall that absence of @optional implies that at least one such edge must exist. If the has_edge_degree filter is used with a parameter set to 0, that requires the edge to not exist. Therefore, if the @optional is not present in this situation, no valid result sets can be produced, and the resulting query will return no results.

• Must be on a vertex field that is not the root vertex of the query.
• Tagged values are not supported as parameters for this filter.
• If the runtime parameter for this operator can be 0, it is strongly recommended to also apply @optional to the vertex field being filtered (see N.B. above for details).

Type coercions are operations that create a new scope whose type is different than the type of the enclosing scope of the coercion -- they coerce the enclosing scope into a different type. Type coercions are represented with GraphQL inline fragments.

{
    Species {
        name @output(out_name: "species_name")
        out_Species_Eats {
            ... on Food {
                name @output(out_name: "food_name")
            }
        }
    }
}

Here, the out_Species_Eats vertex field is of the FoodOrSpecies union type. To proceed with the query, the user must choose which of the types in the FoodOrSpecies union to use. In this example, ... on Food indicates that the Food type was chosen, and any vertices at that scope that are not of type Food are filtered out and discarded.

{
    Species {
        name @output(out_name: "species_name")
        out_Entity_Related {
            ... on Species {
                name @output(out_name: "food_name")
            }
        }
    }
}

In this query, the out_Entity_Related is of Entity type. However, the query only wants to return results where the related entity is a Species, which ... on Species ensures is the case.

The compiler supports the standard GraphQL meta field __typename, which returns the runtime type of the scope where the field is found. Assuming the GraphQL schema matches the database's schema, the runtime type will always be a subtype of (or exactly equal to) the static type of the scope determined by the GraphQL type system. Below, we provide an example query in which the runtime type is a subtype of the static type, but is not equal to it.

The __typename field is treated as a property field of type String, and supports all directives that can be applied to any other property field.

{
    Entity {
        __typename @output(out_name: "entity_type")
        name @output(out_name: "entity_name")
    }
}

This query returns one row for each Entity vertex. The scope in which __typename appears is of static type Entity. However, Animal is a type of Entity, as are Species, Food, and others. Vertices of all subtypes of Entity will therefore be returned, and the entity_type column that outputs the __typename field will show their runtime type: Animal, Species, Food, etc.

This section assumes that the reader is familiar with the way schemas work in the reference implementation of GraphQL.

The GraphQL schema used with the compiler must contain the custom directives and custom Date and DateTime scalar types defined by the compiler:

directive @recurse(depth: Int!) on FIELD

directive @filter(value: [String!]!, op_name: String!) on FIELD | INLINE_FRAGMENT

directive @tag(tag_name: String!) on FIELD

directive @output(out_name: String!) on FIELD

directive @output_source on FIELD

directive @optional on FIELD

directive @fold on FIELD

scalar DateTime

scalar Date

If constructing the schema programmatically, one can simply import the the Python object representations of the custom directives and the custom types:

from graphql_compiler import DIRECTIVES  # the list of custom directives
from graphql_compiler import GraphQLDate, GraphQLDateTime  # the custom types

Since the GraphQL and OrientDB type systems have different rules, there is no one-size-fits-all solution to writing the GraphQL schema for a given database schema. However, the following rules of thumb are useful to keep in mind:

• Generally, represent OrientDB abstract classes as GraphQL interfaces. In GraphQL's type system, GraphQL interfaces cannot inherit from other GraphQL interfaces.
• Generally, represent OrientDB non-abstract classes as GraphQL types, listing the GraphQL interfaces that they implement. In GraphQL's type system, GraphQL types cannot inherit from other GraphQL types.
• Inheritance relationships between two OrientDB non-abstract classes, or between two OrientDB abstract classes, introduce some difficulties in GraphQL. When modelling your data in OrientDB, it's best to avoid such inheritance if possible.
• If it is impossible to avoid having two non-abstract OrientDB classes A and B such that B inherits from A, you have two options:
  • You may choose to represent the A OrientDB class as a GraphQL interface, which the GraphQL type corresponding to B can implement. In this case, the GraphQL schema preserves the inheritance relationship between A and B, but sacrifices the representation of any inheritance relationships A may have with any OrientDB superclasses.
  • You may choose to represent both A and B as GraphQL types. The tradeoff in this case is exactly the opposite from the previous case: the GraphQL schema sacrifices the inheritance relationship between A and B, but preserves the inheritance relationships of A with its superclasses. In this case, it is recommended to create a GraphQL union type A | B, and to use that GraphQL union type for any vertex fields that in OrientDB would be of type A.
• If it is impossible to avoid having two abstract OrientDB classes A and B such that B inherits from A, you similarly have two options:
  • You may choose to represent B as a GraphQL type that can implement the GraphQL interface corresponding to A. This makes the GraphQL schema preserve the inheritance relationship between A and B, but sacrifices the ability for other GraphQL types to inherit from B.
  • You may choose to represent both A and B as GraphQL interfaces, sacrificing the schema's representation of the inheritance between A and B, but allowing GraphQL types to inherit from both A and B. If necessary, you can then create a GraphQL union type A | B and use it for any vertex fields that in OrientDB would be of type A.
• It is legal to fully omit classes and fields that are not representable in GraphQL. The compiler currently does not support OrientDB's EmbeddedMap type nor embedded non-primitive typed fields, so such fields can simply be omitted in the GraphQL representation of their classes. Alternatively, the entire OrientDB class and all edges that may point to it may be omitted entirely from the GraphQL schema.

Since the GraphQL compiler can target multiple different query languages, each with its own behaviors and limitations, the execution model must also be defined as a function of the compilation target language. While we strive to minimize the differences between compilation targets, some differences are unavoidable.

The compiler abides by the following principles:

• When the database is queried with a compiled query string, its response must always be in the form of a list of results.
• The precise format of each such result is defined by each compilation target separately.
  • Both gremlin and MATCH return data in a tabular format, where each result is a row of the table, and fields marked for output are columns.
  • However, future compilation targets may have a different format. For example, each result may appear in the nested tree format used by the standard GraphQL specification.
• Each such result must satisfy all directives and types in its corresponding GraphQL query.
• The returned list of results is not guaranteed to be complete!
  • In other words, there may have been additional result sets that satisfy all directives and types in the corresponding GraphQL query, but were not returned by the database.
  • However, compilation target implementations are encouraged to return complete results if at all practical. The MATCH compilation target is guaranteed to produce complete results.

To explain the completeness of returned results in more detail, assume the database contains the following example graph:

a  ---->_ x
|____   /|
    _|_/
   / |____
  /      \/
b  ----> y

Let a, b, x, y be the values of the name property field of four vertices. Let the vertices named a and b be of type S, and let x and y be of type T. Let vertex a be connected to both x and y via directed edges of type E. Similarly, let vertex b also be connected to both x and y via directed edges of type E.

Consider the GraphQL query:

{
    S {
        name @output(out_name: "s_name")
        out_E {
            name @output(out_name: "t_name")
        }
    }
}

Between the data in the database and the query's structure, it is clear that combining any of a or b with any of x or y would produce a valid result. Therefore, the complete result list, shown here in JSON format, would be:

[
    {"s_name": "a", "t_name": "x"},
    {"s_name": "a", "t_name": "y"},
    {"s_name": "b", "t_name": "x"},
    {"s_name": "b", "t_name": "y"},
]

This is precisely what the MATCH compilation target is guaranteed to produce. The remainder of this section is only applicable to the gremlin compilation target. If using MATCH, all of the queries listed in the remainder of this section will produce the same, complete result list.

Since the gremlin compilation target does not guarantee a complete result list, querying the database using a query string generated by the gremlin compilation target will produce only a partial result list resembling the following:

[
    {"s_name": "a", "t_name": "x"},
    {"s_name": "b", "t_name": "x"},
]

Due to limitations in the underlying query language, gremlin will by default produce at most one result for each of the starting locations in the query. The above GraphQL query started at the type S, so each s_name in the returned result list is therefore distinct. Furthermore, there is no guarantee (and no way to know ahead of time) whether x or y will be returned as the t_name value in each result, as they are both valid results.

Users may apply the @output_source directive on the last scope of the query to alter this behavior:

{
    S {
        name @output(out_name: "s_name")
        out_E @output_source {
            name @output(out_name: "t_name")
        }
    }
}

Rather than producing at most one result for each S, the query will now produce at most one result for each distinct value that can be found at out_E, where the directive is applied:

[
    {"s_name": "a", "t_name": "x"},
    {"s_name": "a", "t_name": "y"},
]

Conceptually, applying the @output_source directive makes it as if the query were written in the opposite order:

{
    T {
        name @output(out_name: "t_name")
        in_E {
            name @output(out_name: "s_name")
        }
    }
}

Including an optional statement in GraphQL has no performance issues on its own, but if you continue expanding vertex fields within an optional scope, there may be significant performance implications.

Going forward, we will refer to two different kinds of @optional directives.

• A "simple" optional is a vertex with an @optional directive that does not expand any vertex fields within it. For example:

{
    Animal {
        name @output(out_name: "name")
        in_Animal_ParentOf @optional {
            name @output(out_name: "parent_name")
        }
    }
}

OrientDB MATCH currently allows the last step in any traversal to be optional. Therefore, the equivalent MATCH traversal for the above GraphQL is as follows:

SELECT
    Animal___1.name as `name`,
    Animal__in_Animal_ParentOf___1.name as `parent_name`
FROM (
    MATCH {
        class: Animal,
        as: Animal___1
    }.in('Animal_ParentOf') {
        as: Animal__in_Animal_ParentOf___1
    }
    RETURN $matches
)

• A "compound" optional is a vertex with an @optional directive which does expand vertex fields within it. For example:

{
    Animal {
        name @output(out_name: "name")
        in_Animal_ParentOf @optional {
            name @output(out_name: "parent_name")
            in_Animal_ParentOf {
                name @output(out_name: "grandparent_name")
            }
        }
    }
}

Currently, this cannot represented by a simple MATCH query. Specifically, the following is NOT a valid MATCH statement, because the optional traversal follows another edge:

-- NOT A VALID QUERY
SELECT
    Animal___1.name as `name`,
    Animal__in_Animal_ParentOf___1.name as `parent_name`
FROM (
    MATCH {
        class: Animal,
        as: Animal___1
    }.in('Animal_ParentOf') {
        optional: true,
        as: Animal__in_Animal_ParentOf___1
    }.in('Animal_ParentOf') {
        as: Animal__in_Animal_ParentOf__in_Animal_ParentOf___1
    }
    RETURN $matches
)

Instead, we represent a compound optional by taking an union (UNIONALL) of two distinct MATCH queries. For instance, the GraphQL query above can be represented as follows:

SELECT EXPAND($final_match)
LET
    $match1 = (
        SELECT
            Animal___1.name AS `name`
        FROM (
            MATCH {
                class: Animal,
                as: Animal___1,
                where: (
                    (in_Animal_ParentOf IS null)
                    OR
                    (in_Animal_ParentOf.size() = 0)
                ),
            }
        )
    ),
    $match2 = (
        SELECT
            Animal___1.name AS `name`,
            Animal__in_Animal_ParentOf___1.name AS `parent_name`
        FROM (
            MATCH {
                class: Animal,
                as: Animal___1
            }.in('Animal_ParentOf') {
                as: Animal__in_Animal_ParentOf___1
            }.in('Animal_ParentOf') {
                as: Animal__in_Animal_ParentOf__in_Animal_ParentOf___1
            }
        )
    ),
    $final_match = UNIONALL($match1, $match2)

In the first case where the optional edge is not followed, we have to explicitly filter out all vertices where the edge could have been followed. This is to eliminate duplicates between the two MATCH selections.

The previous example is not exactly how we implement compound optionals (we also have SELECT statements within $match1 and $match2), but it illustrates the the general idea.

If we have many compound optionals in the given GraphQL, the above procedure results in the union of a large number of MATCH queries. Specifically, for n compound optionals, we generate 2ⁿ different MATCH queries. For each of the 2ⁿ subsets S of the n optional edges:

• We remove the @optional restriction for each traversal in S.
• For each traverse t in the complement of S, we entirely discard t along with all the vertices and directives within it, and we add a filter on the previous traverse to ensure that the edge corresponding to t does not exist.

Therefore, we get a performance penalty that grows exponentially with the number of compound optional edges. This is important to keep in mind when writing queries with many optional directives.

This compilation parameter is a workaround for the limitations of the GraphQL and Gremlin type systems:

• GraphQL does not allow type to inherit from another type, only to implement an interface.
• Gremlin does not have first-class support for inheritance at all.

Assume the following GraphQL schema:

type Animal {
    name: String
}

type Cat {
    name: String
}

type Dog {
    name: String
}

union AnimalCatDog = Animal | Cat | Dog

type Foo {
    adjacent_animal: AnimalCatDog
}

An appropriate type_equivalence_hints value here would be { Animal: AnimalCatDog }. This lets the compiler know that the AnimalCatDog union type is implicitly equivalent to the Animal type, as there are no other types that inherit from Animal in the database schema. This allows the compiler to perform accurate type coercions in Gremlin, as well as optimize away type coercions across edges of union type if the coercion is coercing to the union's equivalent type.

Setting type_equivalence_hints = { Animal: AnimalCatDog } during compilation would enable the use of a @fold on the adjacent_animal vertex field of Foo:

{
    Foo {
        adjacent_animal @fold {
            ... on Animal {
                name @output(out_name: "name")
            }
        }
    }
}

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Copyright 2017-present Kensho Technologies, LLC. The present date is determined by the timestamp of the most recent commit in the repository.