PostgreSQL-CrossTab或带有可变数字列的数据透视表

Question

I need to make a query using pivot or crosstab with columns variables. Is possible? I have the following table:

Data_Id ! Description_Column | Value
--------+--------------------+-------
1       ! Column1            ! value1
1       ! Column2            ! value2 
1       ! Column3            ! value3
1       ! Column4            ! value4
2       ! Column1            ! value5
2       ! Column2            ! value6
2       ! Column3            ! value7
2       ! Column4            ! value8

I will need get

Data_Id ! Column1 ! Column2 ! Column3 ! Column4
1       ! value1  ! value2  ! value3  ! value4
2       ! value5  ! value6  ! value7  ! value8

But sometimes i will have:

Data_Id ! Description_Column | Value
--------+--------------------+-------
3       ! Column1            ! value1
3       ! Column2            ! value2 
4       ! Column1            ! value5
4       ! Column2            ! value6

I will need get

Data_Id ! Column1 ! Column2 
3       ! value1  ! value2  
4       ! value5  ! value6  

Obs. The rows that I'll need will be selected through a Join. So I will have the number of variable columns.

Thanks for help.

Answer 1

You have to remember that the SQL planner needs to know at plan time what a return row looks like in rough terms. So you can't do this directly.

The first is you can create a function which does what you are trying to do and returns the result in a refcursor. That's probably the easiest way to do it. Use the crosstab() function from the tablefunc contrib module/extension. You should be able to generate the SQL query to run based on querying the data but that requires in-function dynamic SQL and again you'd have to return a refcursor.