[SOLVED] Generate one additional row per array element

Generate one additional row per array element

Using PostgreSQL 9.6, I'm trying to insert rows into a table map, generated from another table tbl.

tbl

name	in1	in2
a	1
b	2	{3}
c	4	{5,6}
d	7	{8,9,10}

Should result in this:

map

name	out1	out2
a	1	1
b	2	2
b	2	3
c	4	4
c	4	5
c	4	6
d	7	7
d	7	8
d	7	9
d	7	10

in1 and each value in the array in2 should get entries for user name. I can't figure out how best to iterate through the array, whether it's a subselect, or a built-in operator.

So far I have:

INSERT INTO map(name, out1, out2)
SELECT (name, in1, in1)
FROM
    tbl;

UPDATE map
SET out2 = 
    (loop possibly of in2?)
FROM
    tbl t
WHERE
     name = t.name;

Solution

Simply:

INSERT INTO map (name, out1, out2)
SELECT pname, in1, unnest(in1 || in2)
FROM   tbl

Assuming this table definition:

CREATE TABLE tbl (
  pname text PRIMARY KEY
, in1   integer NOT NULL
, in2   integer[]  --  can be NULL?
);

fiddle

Notably, in2 is an integer array (integer[]). Else, cast with in2::int[] - you have valid array literals on display.

About the element-to-array concatenation in in1 || in2, see:

How can the concatenatation of a text and a NULL value return a non-null result?

in1 and in2 can be NULL, but consider corner cases demonstrated in my fiddle. You may want to move unnest() to a LATERAL subquery to steer corner cases. See:

Flattening a relation with an array to emit one row per array entry

Careful with multiple set-returning functions like unnest() in the SELECT list, especially before Postgres 10. (Not an issue with just one.) See: