Pivot with column name in Postgres - sql

Question

I have the following table tbl:
column1 | column2 | column 3
-----------------------------------
1 | 'value1' | 3
2 | 'value2' | 4
How to do "pivot" with column names to produce output like:
column1 | 1 | 2
column2 | 'value1' |'value2'
column3 | 3 | 4

Answer 1

As has been commented, the issue of data types is undefined in the question.
If you are OK with all result columns being type text (every data type can be converted to text), you can use one of these:
Plain SQL
WITH cte AS (
SELECT nu.*
FROM tbl t
, LATERAL (
VALUES
(1, t.column1::text)
, (2, t.column2)
, (3, t.column3::text)
) nu(rn, c)
)
SELECT *
FROM (TABLE cte OFFSET 0 LIMIT 3) c1
JOIN (TABLE cte OFFSET 3 LIMIT 3) c2 USING (rn);
The same with useful column names:
WITH cte AS (
SELECT nu.*
FROM tbl t
, LATERAL (
VALUES
('column1', t.column1::text)
, ('column2', t.column2)
, ('column3', t.column3::text)
) nu(rn, c)
)
SELECT * FROM (
SELECT *
FROM (TABLE cte OFFSET 0 LIMIT 3) c1
JOIN (TABLE cte OFFSET 3 LIMIT 3) c2 USING (rn)
) t (key, row1, row2);
Works in any modern version of Postgres.
The SQL string has to be adapted to the number of rows and columns. See fiddles below!
Using a document type as stepping stone
Makes for shorter code.
With many rows and many columns, performance of the SQL solution may scale better because the intermediate derived table is smaller.
(The thread is limited as you can't have more than ~ 1600 table columns in Postgres.)
Since everything is converted to text anyway, hstore seems most efficient. See:
Key value pair in PostgreSQL
SELECT key
, arr[1] AS row1
, arr[2] AS row2
FROM (
SELECT x.key, array_agg(x.value) AS arr
FROM tbl t, each(hstore(t)) x
GROUP BY 1
) sub
ORDER BY 1;
Technically speaking we would have to enforce the right sort order when in array_agg(), but that should work without explicit ORDER BY. To be absolutely sure you can add one: array_agg(x.value ORDER BY t.ctid) Using ctid for lack of information.
You can do the same with JSON functions in (Postgres 9.3+). Just replace each(hstore(t) with json_each_text(row_to_json(t). The rest is identical.
These fiddles demonstrate how to scale each query:
Original example with 2 rows of 3 columns:
db<>fiddle here
Scaled up to 3 rows of 4 columns:
db<>fiddle here