I am trying to create crosstab
queries in PostgreSQL such that it automatically generates the crosstab
columns instead of hardcoding it. I have written a function that dynamically generates the column list that I need for my crosstab
query. The idea is to substitute the result of this function in the crosstab
query using dynamic sql.
I know how to do this easily in SQL Server, but my limited knowledge of PostgreSQL is hindering my progress here. I was thinking of storing the result of function that generates the dynamic list of columns into a variable and use that to dynamically build the sql query. It would be great if someone could guide me regarding the same.
-- Table which has be pivoted
CREATE TABLE test_db
(
kernel_id int,
key int,
value int
);
INSERT INTO test_db VALUES
(1,1,99),
(1,2,78),
(2,1,66),
(3,1,44),
(3,2,55),
(3,3,89);
-- This function dynamically returns the list of columns for crosstab
CREATE FUNCTION test() RETURNS TEXT AS '
DECLARE
key_id int;
text_op TEXT = '' kernel_id int, '';
BEGIN
FOR key_id IN SELECT DISTINCT key FROM test_db ORDER BY key LOOP
text_op := text_op || key_id || '' int , '' ;
END LOOP;
text_op := text_op || '' DUMMY text'';
RETURN text_op;
END;
' LANGUAGE 'plpgsql';
-- This query works. I just need to convert the static list
-- of crosstab columns to be generated dynamically.
SELECT * FROM
crosstab
(
'SELECT kernel_id, key, value FROM test_db ORDER BY 1,2',
'SELECT DISTINCT key FROM test_db ORDER BY 1'
)
AS x (kernel_id int, key1 int, key2 int, key3 int); -- How can I replace ..
-- .. this static list with a dynamically generated list of columns ?
You can use the provided C function crosstab_hash
for this.
The manual is not very clear in this respect. It's mentioned at the end of the chapter on crosstab()
with two parameters:
You can create predefined functions to avoid having to write out the result column names and types in each query. See the examples in the previous section. The underlying C function for this form of
crosstab
is namedcrosstab_hash
.
For your example:
CREATE OR REPLACE FUNCTION f_cross_test_db(text, text)
RETURNS TABLE (kernel_id int, key1 int, key2 int, key3 int)
AS '$libdir/tablefunc','crosstab_hash' LANGUAGE C STABLE STRICT;
Call:
SELECT * FROM f_cross_test_db(
'SELECT kernel_id, key, value FROM test_db ORDER BY 1,2'
,'SELECT DISTINCT key FROM test_db ORDER BY 1');
Note that you need to create a distinct crosstab_hash
function for every crosstab
function with a different return type.
Related:
Your function to generate the column list is rather convoluted, the result is incorrect (int
missing after kernel_id
), it can be replaced with this SQL query:
SELECT 'kernel_id int, '
|| string_agg(DISTINCT key::text, ' int, ' ORDER BY key::text)
|| ' int, DUMMY text'
FROM test_db;
And it cannot be used dynamically anyway.