Shapes​

Shaping Query Results​

At the end of the day, EdgeQL has two jobs that are similar, yet distinct:

1. Express the values that we want computed.

2. Arrange the values into a particular shape that we want.

Consider the task of getting “names of users and all of the friends’ names associated with the given user” in a database defined by the following schema:

type User {
    required name: str;
    multi friends: User;
}

If we only concern ourselves with getting the values, then a reasonable solution to this might be:

db> 

select (User.name, User.friends.name ?? '');

{
  ('Alice', 'Cameron'),
  ('Alice', 'Dana'),
  ('Billie', 'Dana'),
  ('Cameron', ''),
  ('Dana', 'Alice'),
  ('Dana', 'Billie'),
  ('Dana', 'Cameron'),
}

This particular solution is very similar to what one might get using SQL. It’s equivalent to a table with “user name” and “friend name” columns. It gets the job done, albeit with some redundant repeating of “user names”.

We can improve things a little and reduce the repetition by aggregating all the friend names into an array:

db> 

select (User.name, array_agg(User.friends.name));

{
  ('Alice', ['Cameron', 'Dana']),
  ('Billie', ['Dana']),
  ('Cameron', []),
  ('Dana', ['Alice', 'Billie', 'Cameron']),
}

This achieves a couple of things: it’s easier to see which friends belong to which user and we no longer need the placeholder '' for those users who don’t have friends.

The recommended way to get this information in EdgeDB, however, is to use shapes, because they mimic the structure of the data and the output:

db> 
... 
... 
... 
... 
... 

select User {
    name,
    friends: {
        name
    }
};

{
  default::User {
    name: 'Alice',
    friends: {
      default::User {name: 'Cameron'},
      default::User {name: 'Dana'},
    },
  },
  default::User {name: 'Billie', friends: {default::User {name: 'Dana'}}},
  default::User {name: 'Cameron', friends: {}},
  default::User {
    name: 'Dana',
    friends: {
      default::User {name: 'Alice'},
      default::User {name: 'Billie'},
      default::User {name: 'Cameron'},
    },
  },
}

So far the expression for the data that we wanted was also acceptable for structuring the output, but what if that’s not the case? Let’s add a condition and only show those users who have friends with either the letter “i” or “o” in their names:

db> 
... 
... 
... 
... 
... 

select User {
    name,
    friends: {
        name
    }
} filter .friends.name ilike '%i%' or .friends.name ilike '%o%';

{
  default::User {
    name: 'Alice',
    friends: {
      default::User {name: 'Cameron'},
      default::User {name: 'Dana'},
    },
  },
  default::User {
    name: 'Dana',
    friends: {
      default::User {name: 'Alice'},
      default::User {name: 'Billie'},
      default::User {name: 'Cameron'},
    },
  },
}

That filter is getting a bit bulky, so perhaps we can just factor these flags out as part of the shape’s computed properties:

db> 
... 
... 
... 
... 
... 
... 
... 

select User {
    name,
    friends: {
        name
    },
    has_i := .friends.name ilike '%i%',
    has_o := .friends.name ilike '%o%',
} filter .has_i or .has_o;

{
  default::User {
    name: 'Alice',
    friends: {
      default::User {name: 'Cameron'},
      default::User {name: 'Dana'},
    },
    has_i: {false, false},
    has_o: {true, false},
  },
  default::User {
    name: 'Dana',
    friends: {
      default::User {name: 'Alice'},
      default::User {name: 'Billie'},
      default::User {name: 'Cameron'},
    },
    has_i: {true, true, false},
    has_o: {false, false, true},
  },
}

It looks like this refactoring came at the cost of putting extra things into the output. In this case we don’t want our intermediate calculations to actually show up in the output, so what can we do? In EdgeDB the output structure is determined only by the expression appearing in the top-level select. This means that we can move our intermediate calculations into the with block:

db> 
... 
... 
... 
... 
... 
... 
... 
... 
... 
... 
... 

with U := (
    select User {
        has_i := .friends.name ilike '%i%',
        has_o := .friends.name ilike '%o%',
    }
)
select U {
    name,
    friends: {
        name
    },
} filter .has_i or .has_o;

{
  default::User {
    name: 'Alice',
    friends: {
      default::User {name: 'Cameron'},
      default::User {name: 'Dana'},
    },
  },
  default::User {
    name: 'Dana',
    friends: {
      default::User {name: 'Alice'},
      default::User {name: 'Billie'},
      default::User {name: 'Cameron'},
    },
  },
}

This way we can use has_i and has_o in our query without leaking them into the output.

General Shaping Rules​

In EdgeDB typically all shapes appearing in the top-level select should be reflected in the output. This also applies to shapes no matter where and how they are nested. Aside from other shapes, this includes nesting in arrays:

db> 

select array_agg(User {name});

{
  [
    default::User {name: 'Alice'},
    default::User {name: 'Billie'},
    default::User {name: 'Cameron'},
    default::User {name: 'Dana'},
  ],
}

… or tuples:

db> 

select enumerate(User {name});

{
  (0, default::User {name: 'Alice'}),
  (1, default::User {name: 'Billie'}),
  (2, default::User {name: 'Cameron'}),
  (3, default::User {name: 'Dana'}),
}

You can safely access a tuple element and expect the output shape to be intact:

db> 

select enumerate(User{name}).1;

{
  default::User {name: 'Alice'},
  default::User {name: 'Billie'},
  default::User {name: 'Cameron'},
  default::User {name: 'Dana'},
}

Accessing array elements or working with slices also preserves output shape and is analogous to using offset and limit when working with sets:

db> 

select array_agg(User {name})[2];

{default::User {name: 'Cameron'}}

Losing Shapes​

There are some situations where shape information gets completely or partially discarded. Any such operation also prevents the altered shape from appearing in the output altogether.

In order for the shape to be preserved, the original expression type must be preserved. This means that union can alter the shape, because the result of a union is a union type. So you can still refer to the common properties, but not to the properties that appeared in the shape.

As mentioned above, since union potentially alters the expression shape it never preserves output shape, even when the underlying type wasn’t altered:

db> 

select User{name} union User{name};

{
  default::User {id: 7769045a-27bf-11ec-94ea-3f6c0ae59eb3},
  default::User {id: 7b42ed20-27bf-11ec-94ea-7700ec77834e},
  default::User {id: 7fcedbc4-27bf-11ec-94ea-73dcb6f297a4},
  default::User {id: 82f52646-27bf-11ec-94ea-3718ffb8dd15},
  default::User {id: 7769045a-27bf-11ec-94ea-3f6c0ae59eb3},
  default::User {id: 7b42ed20-27bf-11ec-94ea-7700ec77834e},
  default::User {id: 7fcedbc4-27bf-11ec-94ea-73dcb6f297a4},
  default::User {id: 82f52646-27bf-11ec-94ea-3718ffb8dd15},
}

Listing several items inside a set { ... } functions identically to a union and so will also produce a union type and remove shape from output.

Another subtle way for a type union to remove the shape from the output is by the ?? and the if..else operators. Both of them determine the result type as the union of the left and right operands:

db> 

select <User>{} ?? User {name};

{
  default::User {id: 7769045a-27bf-11ec-94ea-3f6c0ae59eb3},
  default::User {id: 7b42ed20-27bf-11ec-94ea-7700ec77834e},
  default::User {id: 7fcedbc4-27bf-11ec-94ea-73dcb6f297a4},
  default::User {id: 82f52646-27bf-11ec-94ea-3718ffb8dd15},
}

Shapes survive array creation (either via array_agg() or by using [ ... ]), but they follow the same rules as for union for array concatenation. Basically the element type of the resulting array must be a union type and thus all shape information is lost:

db> 

select array_agg(User{name}) ++ array_agg(User{name});

{
  [
    default::User {id: 7769045a-27bf-11ec-94ea-3f6c0ae59eb3},
    default::User {id: 7b42ed20-27bf-11ec-94ea-7700ec77834e},
    default::User {id: 7fcedbc4-27bf-11ec-94ea-73dcb6f297a4},
    default::User {id: 82f52646-27bf-11ec-94ea-3718ffb8dd15},
    default::User {id: 7769045a-27bf-11ec-94ea-3f6c0ae59eb3},
    default::User {id: 7b42ed20-27bf-11ec-94ea-7700ec77834e},
    default::User {id: 7fcedbc4-27bf-11ec-94ea-73dcb6f297a4},
    default::User {id: 82f52646-27bf-11ec-94ea-3718ffb8dd15},
  ],
}