Re: query with pg_trgm sometimes very slow

CREATE INDEX trgm_adresse ON adressen.adresse USING gist (normalize_string((btrim((((((((normalize_string((((COALESCE((vorname)::text, ''::text) || ' '::text) ||   (name1)::text))::character varying, (-1)))::text || ' '::text) || (normalize_string((COALESCE((strasse)::text, ''::text))::character varying, (-2)))::text) || ' '::text) || (plz)::text) || ' '::text) || (normalize_string((COALESCE((ort)::text, ''::text))::character varying, (-3)))::text)))::character varying) gist_trgm_ops);

When such a slow query is running, 'top' shows nearly '100 % wait' and 'iotop' shows 3 - 8 MB/sec disk read by a process
    postgres: vb vb 10.128.96.25(60435) FETCH

Also the postgres log, which was told to log every task longer than 5000 ms, shows

    2015-09-02 13:44:48 CEST [25237-1] vb@vb LOG:  duration: 55817.191 ms  execute <unnamed>: FETCH FORWARD 4096 IN "py:0xa2d61f6c"

Since I never used a FETCH command in my life, this must be used by pg_trgm or something inside it (gin, gist etc.)