Dynamic column names in a postgres crosstab query - postgresql

Question

I am trying to pivot the data in a query in postgres. The query I am currently using is as follows
SELECT
product_number,
month,
sum(quantity)
FROM forecasts
WHERE date_trunc('month', extract_date) = date_trunc('month', current_date)
GROUP BY product_number, month
ORDER BY product_number, month;
The output of the query is something like what is shown below where each product will have 13 months of data.
+--------+------------+----------+
| Number | Month | Quantity |
+--------+------------+----------+
| 1 | 2016-10-01 | 7592 |
| 1 | 2016-11-01 | 6796 |
| 1 | 2016-12-01 | 6512 |
| 1 | 2017-01-01 | 6160 |
| 1 | 2017-02-01 | 6475 |
| 1 | 2017-03-01 | 6016 |
| 1 | 2017-04-01 | 6616 |
| 1 | 2017-05-01 | 6536 |
| 1 | 2017-06-01 | 6256 |
| 1 | 2017-07-01 | 6300 |
| 1 | 2017-08-01 | 5980 |
| 1 | 2017-09-01 | 5872 |
| 1 | 2017-10-01 | 5824 |
+--------+------------+----------+
I am trying to pivot the data so that it looks something like
+--------+-----------+-----------+-----------+----------+-----+
| Number | 2016-10-1 | 2016-11-1 | 2016-12-1 | 2017-1-1 | ... |
+--------+-----------+-----------+-----------+----------+-----+
| 1 | 100 | 100 | 200 | 250 | ... |
| ... | | | | | |
+--------+-----------+-----------+-----------+----------+-----+
Where all the data for each product is shown in a row for the 13 months.
I tried using a basic crosstab query
SELECT *
FROM
crosstab('SELECT product_number, month::TEXT, sum(quantity)
FROM forecasts
WHERE date_trunc(''month'', extract_date) = date_trunc(''month'', ''2016-10-1''::DATE)
GROUP BY product_number, month
ORDER BY product_number, month')
As mthreport(product_number text, m0 DATE, m1 DATE, m2 DATE,
m3 DATE, m4 DATE, m5 DATE, m6 DATE,
m7 DATE, m8 DATE, m9 DATE, m10 DATE,
m11 DATE, m12 DATE, m13 DATE)
But I get the following error
ERROR: invalid return type Detail: SQL rowid datatype does not match return rowid datatype.
If the column name were set in the crosstab i.e. if I could define and put the names into the crosstab output this works, but since the dates keep changing I am not sure how to define them
I think I missing something very basic here. Any help would be really appreciated.

Answer 1

Hoping, i have understood your problem correctly.
Column m1, m2 .. m13 are not of date type. These columns will contain sum of quantity. So, data type will be same as sum(quantity).
I think below query will solve your problem
SELECT *
FROM
crosstab($$SELECT product_number, month, sum(quantity)::bigint
FROM forecasts
GROUP BY product_number, month
ORDER BY product_number, month$$)
As mthreport(product_number int, m0 bigint, m1 bigint, m2 bigint,
m3 bigint, m4 bigint, m5 bigint, m6 bigint,
m7 bigint, m8 bigint, m9 bigint, m10 bigint,
m11 bigint, m12 bigint , m13 bigint)