BEGIN;

CREATE TEMPORARY TABLE old_table (
    id  integer NOT NULL PRIMARY KEY,
    a   varchar NOT NULL,
    b   varchar NOT NULL,
    c   varchar NOT NULL,
    abc_id  integer NOT NULL DEFAULT -1
)
ON COMMIT DROP;

CREATE TEMPORARY TABLE new_table (
    id  integer NOT NULL PRIMARY KEY,
    a   varchar NOT NULL,
    b   varchar NOT NULL,
    c   varchar NOT NULL,
    UNIQUE(a, b, c)
)
ON COMMIT DROP;

CREATE TEMPORARY TABLE new_table_temp (
    LIKE new_table INCLUDING ALL,
    old_ids integer[]
)
ON COMMIT DROP;

EXPLAIN WITH ins AS (
    INSERT INTO new_table (a, b, c)
    SELECT DISTINCT a, b, c
    FROM old_table
    RETURNING *
    )
UPDATE old_table
SET abc_id = ins.id
FROM ins
WHERE
    ins.a = old_table.a
AND ins.b = old_table.b
AND ins.c = old_table.c;