[postgis-users] pl/pgsql question: dynamic access to record.column

[Dylan Beaudette]

Hi,

I am learning how to use PL/PGSQL and have come upon something that I just 
don't seem to be able to solve on my own. 

I have a function, which takes a column name as an argument. A FOR loop 
iterates over rows, using a RECORD datatype to store each iteration of the 
loop. 

If I hard-code in a column name into some calculation, things work fine. 
However, I cannot seem to figure out how to _dynamically_ access a column 
from a RECORD datatype:

-- this works fine : 'field_pct_clay' is a hard-coded column name
DEFINE
hz_record RECORD;
wt_column FLOAT;
thick FLOAT;
...
wt_column := wt_column + (hz_record.field_pct_clay * thick) ;

-- this does not work: 'the_column' is an argument to the function being 
defined:

wt_column := wt_column + (hz_record.the_column * thick) ;
                                                            ^^^^^^^^^^^^^

obviously, the interpreter tells me that there is no column named 'the_column' 
in the RECORD iterator... 

Searching on google, it seems that a dynamic expression like this needs to be 
evaluated with the EXECUTE keyword.... However something like this does not 
work:

wt_column := EXECUTE 'wt_column + (hz_record.' || quote_ident(the_column) || ' 
* thick)' ;

any tips would be a great help!

thanks!
-- 
Dylan Beaudette
Soils and Biogeochemistry Graduate Group
University of California at Davis
530.754.7341