Basing my answer on a table of the form:
CREATE TABLE tbl (
sl_no int
, username text
, designation text
, salary int
);
Each row results in a new column to return. With a dynamic return type like this, it's hardly possible to make this completely dynamic with a single call to the database. Demonstrating solutions with two steps:
- Generate query
- Execute generated query
Generally, this is limited by the maximum number of columns a table can hold. So not an option for tables with more than 1600 rows (or fewer). Details:
Postgres 9.4+
Dynamic solution with crosstab()
Use the first one if you can. Beats the rest.
SELECT 'SELECT *
FROM crosstab(
$ct$SELECT u.attnum, t.rn, u.val
FROM (SELECT row_number() OVER () AS rn, * FROM '
|| attrelid::regclass || ') t
, unnest(ARRAY[' || string_agg(quote_ident(attname)
|| '::text', ',') || '])
WITH ORDINALITY u(val, attnum)
ORDER BY 1, 2$ct$
) t (attnum bigint, '
|| (SELECT string_agg('r'|| rn ||' text', ', ')
FROM (SELECT row_number() OVER () AS rn FROM tbl) t)
|| ')' AS sql
FROM pg_attribute
WHERE attrelid = 'tbl'::regclass
AND attnum > 0
AND NOT attisdropped
GROUP BY attrelid;
Operating with attnum instead of actual column names. Simpler and faster. Join the result to pg_attribute once more or integrate column names like in the pg 9.3 example.
Generates a query of the form:
SELECT *
FROM crosstab(
$ct$
SELECT u.attnum, t.rn, u.val
FROM (SELECT row_number() OVER () AS rn, * FROM tbl) t
, unnest(ARRAY[sl_no::text,username::text,designation::text,salary::text]) WITH ORDINALITY u(val, attnum)
ORDER BY 1, 2$ct$
) t (attnum bigint, r1 text, r2 text, r3 text, r4 text);
This uses a whole range of advanced features. Just too much to explain.
Simple solution with unnest()
One unnest() can now take multiple arrays to unnest in parallel.
SELECT 'SELECT * FROM unnest(
''{sl_no, username, designation, salary}''::text[]
, ' || string_agg(quote_literal(ARRAY[sl_no::text, username::text, designation::text, salary::text])
|| '::text[]', E'\n, ')
|| E') \n AS t(col,' || string_agg('row' || sl_no, ',') || ')' AS sql
FROM tbl;
Result:
SELECT * FROM unnest(
'{sl_no, username, designation, salary}'::text[]
,'{10,Joe,Music,1234}'::text[]
,'{11,Bob,Movie,2345}'::text[]
,'{12,Dave,Theatre,2356}'::text[])
AS t(col,row1,row2,row3,row4);
db<>fiddle here
Old sqlfiddle
Postgres 9.3 or older
Dynamic solution with crosstab()
- Completely dynamic, works for any table. Provide the table name in two places:
SELECT 'SELECT *
FROM crosstab(
''SELECT unnest(''' || quote_literal(array_agg(attname))
|| '''::text[]) AS col
, row_number() OVER ()
, unnest(ARRAY[' || string_agg(quote_ident(attname)
|| '::text', ',') || ']) AS val
FROM ' || attrelid::regclass || '
ORDER BY generate_series(1,' || count(*) || '), 2''
) t (col text, '
|| (SELECT string_agg('r'|| rn ||' text', ',')
FROM (SELECT row_number() OVER () AS rn FROM tbl) t)
|| ')' AS sql
FROM pg_attribute
WHERE attrelid = 'tbl'::regclass
AND attnum > 0
AND NOT attisdropped
GROUP BY attrelid;
Could be wrapped into a function with a single parameter ...
Generates a query of the form:
SELECT *
FROM crosstab(
'SELECT unnest(''{sl_no,username,designation,salary}''::text[]) AS col
, row_number() OVER ()
, unnest(ARRAY[sl_no::text,username::text,designation::text,salary::text]) AS val
FROM tbl
ORDER BY generate_series(1,4), 2'
) t (col text, r1 text,r2 text,r3 text,r4 text);
Produces the desired result:
col r1 r2 r3 r4
-----------------------------------
sl_no 1 2 3 4
username A B C D
designation XYZ RTS QWE HGD
salary 10000 50000 20000 34343
Simple solution with unnest()
SELECT 'SELECT unnest(''{sl_no, username, designation, salary}''::text[] AS col)
, ' || string_agg('unnest('
|| quote_literal(ARRAY[sl_no::text, username::text, designation::text, salary::text])
|| '::text[]) AS row' || sl_no, E'\n , ') AS sql
FROM tbl;
- Slow for tables with more than a couple of columns.
Generates a query of the form:
SELECT unnest('{sl_no, username, designation, salary}'::text[]) AS col
, unnest('{10,Joe,Music,1234}'::text[]) AS row1
, unnest('{11,Bob,Movie,2345}'::text[]) AS row2
, unnest('{12,Dave,Theatre,2356}'::text[]) AS row3
, unnest('{4,D,HGD,34343}'::text[]) AS row4
Same result.
