CREATE TABLE picture_categories
(
  pcid int4 NOT NULL DEFAULT nextval('picture_categories_pcid_seq'::regclass),
  pid int4 NOT NULL,
  cat_id int4,
  CONSTRAINT picture_categories_pkey PRIMARY KEY (pcid),
  CONSTRAINT picture_categories_pid_key UNIQUE (pid, cat_id)
) 
WITHOUT OIDS;
 
CREATE INDEX idx_picture_categories_cat_id ON picture_categories USING btree (cat_id);
CREATE INDEX idx_picture_categories_pid ON picture_categories USING btree (pid);
CREATE INDEX idx_picture_categories_pid_and_cat_id ON picture_categories USING btree (pid, cat_id);
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1. Drop pcid and use idx_picture_categories_pid_and_cat_id as PK
2. Change cat_id to int2 if possible