I was reading through the Postgres documentation on materialized views and in one example they use the following:

SELECT word FROM words ORDER BY word <-> 'caterpiler' LIMIT 10;

I tried to run a similar query on my own database, but I get an error.

[42883] ERROR: operator does not exist: character varying <-> unknown

I (probably) don't need to use the operator, but I'm just curious what it does.


  • I am running the same version as the docs (9.3)
  • I did try Googling the question, but Google just ignores the <-> even if you wrap it in quotes.
  • 1
    Not quite so. I found this question by googling "postgresql <->" – alexkovelsky Nov 2 '18 at 21:55

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:


More details:

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.

| improve this answer | |
  • Thanks, that explains why it didn't work on my database. I see now that the extention is used in the documentation example. – Villiers Strauss Dec 22 '14 at 13:26

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