PostgreSQL Operator – What Does the Vertical Pipe Slash Operator Do?

It's supposed to be the "distance" operator from the additional module pg_trgm.

The manual:

text <-> text real Returns the "distance" between the arguments, that is one minus the similarity() value.

The module has to be installed (once per database) with:

CREATE EXTENSION pg_trgm;

More details:

• How is LIKE implemented?

Theoretically, any user with the necessary privileges could create an operator with that operator name using CREATE OPERATOR - but not in the example you link to.

Supported operators and workaround

The default operator class for GIN indexes on json columns jsonb_ops only supports these operators (per documentaion):

Name        Indexed Data Type   Indexable Operators
...
jsonb_ops   jsonb               ? ?& ?| @>

You can achieve this the other way round: Create a simple IMMUTABLE SQL function using the ? operator, which can be inlined and will use the index just like the operator itself:

CREATE OR REPLACE FUNCTION jb_contains(jsonb, text)
  RETURNS bool AS
'SELECT $1 ? $2' LANGUAGE sql IMMUTABLE;

This works, I tested in Postgres 9.4 ...

Misunderstandings

However, you have been asking the wrong question. There are two basic misconceptions in your question.

1. The jsonb operator ? cannot be used to search for values. Only for keys or array elements. The manual:

   Description:
   Does the string exist as a top-level key within the JSON value?

   You got the wrong operator, the WHERE condition cannot work: WHERE d->'name' ? 'First'

2. The expression index you have doesn't makes sense either way

   CREATE INDEX idx_jsonthings_name ON jsonthings USING GIN ((d->'name'));

   • The expression d->'name' returns a jsonb value. You would need d->>'name' to get the value as text.

   • But that would still be pointless. Since the value of the name key is a simple string, a GIN index (while possible) makes no sense to begin with.

Solutions

You don't need the operator ? - so no workaround either.
Here are two ways that would actually work:

1. Plain GIN index on d and use the "contains" operator @>:

   CREATE INDEX idx_jsonthings_d_gin ON jsonthings USING GIN (d);
   
   SELECT d FROM jsonthings WHERE d @> '{"name":"First"}'
   

   You could even use the more specialized operator class jsonb_path_ops. See:

   • How to get particular object from jsonb array in PostgreSQL?

2. B-tree expression index on d->>'email' and test with good old =:

   CREATE INDEX idx_jsonthings_d_email ON jsonthings ((d->>'email'));
   
   SELECT d FROM jsonthings WHERE d->>'email' = 'First';
   

The second index would be considerably smaller and the query faster.