extracting the domain from an email address

I am searching for a better more efficient way to extract the domain portion of an email address.  The two I have been using are very expensive.

   The one extract I mostly use:  substring(email from '@(.*)$')

       also has an index on the email column which is the full email address:

              btree (email DESC) WHERE email::text = "substring"(email::text, '@(.*)$'::text) AND length(email::text) > 0,

     

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