Create indexes and check constraints on JSON columns

The examples in this section rely on the -> and ->> operators.

Often, when JSON documents are inserted into a table, the table will have just a self-populating surrogate primary key column and a value column, like doc, of data type jsonb. Choosing jsonb allows the use of a broader range of operators and functions, and allows these to execute more efficiently, than does choosing json.

It's most likely that each document will be a JSON object and that all will conform to the same structure definition. (The structure can be defined formally, and externally, by a so-called "JSON schema".) In other words, each object will have the same set of possible key names (but some might be missing) and the same JSON data type for the value for each key. And when a data type is compound, the same notion of common structure definition will apply, extending the notion recursively to arbitrary depth. Here is an example. To reduce clutter, the primary key is not defined to be self-populating.

create table books(k int primary key, doc jsonb not null);

insert into books(k, doc) values
  '{ "ISBN"    : 4582546494267,
     "title"   : "Macbeth",
     "author"  : {"given_name": "William", "family_name": "Shakespeare"},
     "year"    : 1623}'),

  '{ "ISBN"    : 8760835734528,
     "title"   : "Hamlet",
     "author"  : {"given_name": "William", "family_name": "Shakespeare"},
     "year"    : 1603,
     "editors" : ["Lysa", "Elizabeth"] }'),

  '{ "ISBN"    : 7658956876542,
     "title"   : "Oliver Twist",
     "author"  : {"given_name": "Charles", "family_name": "Dickens"},
     "year"    : 1838,
     "genre"   : "novel",
     "editors" : ["Mark", "Tony", "Britney"] }'),
  '{ "ISBN"    : 9874563896457,
     "title"   : "Great Expectations",
     "author"  : {"family_name": "Dickens"},
     "year"    : 1950,
     "genre"   : "novel",
     "editors" : ["Robert", "John", "Melisa", "Elizabeth"] }'),

  '{ "ISBN"    : 8647295405123,
     "title"   : "A Brief History of Time",
     "author"  : {"given_name": "Stephen", "family_name": "Hawking"},
     "year"    : 1988,
     "genre"   : "science",
     "editors" : ["Melisa", "Mark", "John", "Fred", "Jane"] }'),

    "ISBN"     : 6563973589123,
    "year"     : 1989,
    "genre"    : "novel",
    "title"    : "Joy Luck Club",
    "author"   : {"given_name": "Amy", "family_name": "Tan"},
    "editors"  : ["Ruilin", "Aiping"]}');

Some of the rows have some of the keys missing. But the row with "k=6" has every key.

You will probably want at least to know if your corpus contains a non-conformant document and, in some cases, you will want to disallow non-conformant documents. You might want to insist that the ISBN is always defined and is a positive 13-digit number.

You will almost certainly want to retrieve documents, typically not by providing the key, but rather by using predicates on their content—in particular, the primitive values that they contain. You will probably want, also, to project out values of interest.

For example, you might want to see the title and author of books whose publication year is later than 1850.

Of course, then, you will want these queries to be supported by indexes. The alternative, a table scan over a huge corpus where each document is analyzed on the fly to evaluate the selection predicates, would probably perform too poorly.

Check constraints on jsonb columns

Here's how to insist that each JSON document is an object:

alter table books
add constraint books_doc_is_object
check (jsonb_typeof(doc) = 'object');

Here's how to insist that the ISBN is always defined and is a positive 13-digit number:

alter table books
add constraint books_isbn_is_positive_13_digit_number
check (
  (doc->'ISBN') is not null
  jsonb_typeof(doc->'ISBN') = 'number'
  (doc->>'ISBN')::bigint > 0
  length(((doc->>'ISBN')::bigint)::text) = 13

Notice that if the key "ISBN" is missing altogether, then the expression doc->'ISBN' will yield a genuine SQL NULL. But the producer of the document might have decided to represent "No information is available about this book's ISBN" with the special JSON value null for the key "ISBN". (Recall that this special value has its own data type.) This is why it's insufficient to test just that jsonb_typeof() yields number (and therefore not null) for the key "ISBN" and why the separate IS NOT NULL test is done as well.

The high-level point is that YSQL allows you to express a constraint using any expression that can be evaluated by referencing values from a single row. The expression can include a PL/pgSQL function. This allows a constraint to be implemented to insist that the keys in the JSON object are from a known list:

create function top_level_keys_ok(json_obj in jsonb)
  returns boolean
  language plpgsql
  key text;
  legal_keys constant varchar(10)[] := array[
    'ISBN', 'title', 'year', 'genre', 'author', 'editors'];
  for key in (
    if not (key = any (legal_keys)) then
      return false;
    end if;
  end loop;
  return true;

alter table books
add constraint books_doc_keys_OK
check (top_level_keys_ok(doc));

See the account of the jsonb_object_keys().

Indexes on jsonb columns

Proper practice requires that when a table has a surrogate primary key, it must also have a unique, NOT NULL, business key. The obvious candidate for the books table is the value for the "ISBN" key. The NOT NULL rule is already enforced by the "books_isbn_is_positive_13_digit_number" constraint. Uniqueness is enforced in the obvious way:

create unique index books_isbn_unq
on books((doc->>'ISBN') hash);

You might want to support range queries that reference the value for the "year" key like this:

  (doc->>'ISBN')::bigint as year,
  doc->>'title'          as title,
  (doc->>'year')::int    as year
from books
where (doc->>'year')::int > 1850
order by 3;

You'll probably want to support this with an index. And if you realize that the publication year is unknown for a substantial proportion of the books, you will probably want to take advantage of a partial index, thus:

create index books_year on books ((doc->>'year') asc)
where doc->>'year' is not null;