Create Postgres JSONB Index on Array Sub-Object

Question

I have table myTable with a JSONB column myJsonb with a data structure that I want to index like:

{
  "myArray": [
    {
      "subItem": {
        "email": "[email protected]"
      }
    },
    {
      "subItem": {
        "email": "[email protected]"
      }
    }
  ]
}

I want to run indexed queries on email like:

SELECT *
FROM mytable
WHERE '[email protected]' IN (
  SELECT lower(
      jsonb_array_elements(myjsonb -> 'myArray')
      -> 'subItem'
      ->> 'email'
  )
);

How do I create a Postgres JSONB index for that?

Erwin Brandstetter Erwin Brandstetter · Accepted Answer · 2017-12-15T04:03:23

If you don't need the lower() in there, the query can be simple and efficient:

SELECT *
FROM   mytable
WHERE  myjsonb -> 'myArray' @> '[{"subItem": {"email": "[email protected]"}}]'

Supported by a jsonb_path_ops index:

CREATE INDEX mytable_myjsonb_gin_idx ON mytable
USING  gin ((myjsonb -> 'myArray') jsonb_path_ops);

But the match is case-sensitive.

Case-insensitive!

If you need the search to match disregarding case, things get more complex.

You could use this query, similar to your original:

SELECT *
FROM   t
WHERE  EXISTS (
   SELECT 1
   FROM   jsonb_array_elements(myjsonb -> 'myArray') arr
   WHERE  lower(arr #>>'{subItem, email}') = '[email protected]'
   );

But I can't think of a good way to use an index for this.

Instead, I would use an expression index based on a function extracting an array of lower-case emails:

Function:

CREATE OR REPLACE FUNCTION f_jsonb_arr_lower(_j jsonb, VARIADIC _path text[])
  RETURNS jsonb LANGUAGE sql IMMUTABLE AS
'SELECT jsonb_agg(lower(elem #>> _path)) FROM jsonb_array_elements(_j) elem';

Index:

CREATE INDEX mytable_email_arr_idx ON mytable
USING  gin (f_jsonb_arr_lower(myjsonb -> 'myArray', 'subItem', 'email') jsonb_path_ops);

Query:

SELECT *
FROM   mytable 
WHERE  f_jsonb_arr_lower(myjsonb -> 'myArray', 'subItem', 'email') @> '"[email protected]"';

While this works with an untyped string literal or with actual jsonb values, it stops working if you pass text or varchar (like in a prepared statement). Postgres does not know how to cast because the input is ambiguous. You need an explicit cast in this case:

... @> '"[email protected]"'::text::jsonb;

Or pass a simple string without enclosing double quotes and do the conversion to jsonb in Postgres:

... @> to_jsonb('[email protected]'::text);

Related, with more explanation:

Create Postgres JSONB Index on Array Sub-Object

1 Answers

Case-insensitive!