[GENERAL] What is the problem with this query?

Hi, ALL,

draft=# IF NOT EXISTS( SELECT 1 FROM pg_class c, pg_namespace n WHERE
n.oid = c.relnamespace AND c.relname = 'abcattbl_tnam_ownr' AND
n.nspname = 'public' ) THEN CREATE INDEX "abcattbl_tnam_ownr" ON
"abcattbl"("abt_tnam" ASC, "abt_ownr" ASC);
ERROR:  syntax error at or near "IF"
LINE 1: IF NOT EXISTS( SELECT 1 FROM pg_class c, pg_namespace n WHER...
        ^

Is "IF" operator not supported by PostgreSQL?

Thank you.