Using dynamic query + user defined datatype in Postgres - sql

Question

I need a function to normalize my input table features values.
My features table has 9 columns out of which x1,x2...x6 are the input columns I need to scale.
I'm able to do it by using a static query:
create or replace function scale_function()
returns void as $$
declare tav1 features%rowtype; rang1 features%rowtype;
begin
select avg(n),avg(x0),avg(x1),avg(x2),avg(x3),avg(x4),avg(x5),avg(x6),avg(y)
into tav1 from features;
select max(n)-min(n),max(x0)-min(x0),max(x1)-min(x1),max(x2)-min(x2),max(x3)-min(x3),
max(x4)-min(x4),max(x5)-min(x5),max(x6)-min(x6),max(y)-min(y)
into rang1 from features;
update features
set x1= (x1-tav1.x1)/(rang1.x1),x2= (x2-tav1.x2)/(rang1.x2),
x3= (x3-tav1.x3)/(rang1.x3),x4= (x4-tav1.x4)/(rang1.x4),
x5= (x5-tav1.x5)/(rang1.x5),x6= (x6-tav1.x6)/(rang1.x6),
y= (y-tav1.y)/(rang1.y);
return;
end;
$$ language plpgsql;
But now I require a dynamic query to scale n column values i.e., x1,x2...,xn (say I've 200+ columns) in my features table. I'm trying this code but this won't work as there is an issue with a user defined data type:
create or replace function scale_function(n int)
returns void as $$
declare
tav1 features%rowtype;
rang1 features%rowtype;
query1 text :=''; query2 text :='';
begin
for i in 0..n
loop
query1 := query1 ||',avg(x'||i||')';
query2 := query2||',max(x'||i||')-min(x'||i||')';
end loop;
query1 := 'select avg(n)'||query1||',avg(y) into tav1 from features;';
execute query1;
query2 := 'select max(n)-min(n)'||query2||',max(y)-min(y) into rang1 from features;';
execute query2;
update features
set x1= (x1-tav1.x1)/(rang1.x1), ... ,xn=(xn-tav1.xn)/(rang1.xn)
,y= (y-tav1.y)/(rang1.y);
return;
end;
$$ language plpgsql;
Here I'm trying to take the avg() values of the columns into a user-defined rowtype tav1 and have to use that tav1 value to update.
Can any one help me how to update the features table values using dynamic query for 'n' such columns?
************ Error ************
ERROR: column "avg" specified more than once
SQL state: 42701
Context: SQL statement "select avg(n),avg(x0),avg(x1),avg(x2),avg(x3),avg(x4),avg(x5),avg(x6),avg(y) into tav1 from features;"
PL/pgSQL function scale_function(integer) line 12 at EXECUTE statement
I'm using PostgreSQL 9.3.0.

Answer 1

Basic UPDATE
Replace the first query with this much shorter and more efficient single UPDATE command:
UPDATE features
SET (x1,x2,x3,x4,x5,x6, y)
= ((x1 - g.avg1) / g.range1
, (x2 - g.avg2) / g.range2
-- , (x3 - ...
, (y - g.avgy) / g.rangey)
FROM (
SELECT avg(x1) AS avg1, max(x1) - min(x1) AS range1
, avg(x2) AS avg2, max(x2) - min(x2) AS range2
-- , avg(x3) ...
, avg(y) AS avgy, max(y) - min(y) AS rangey
FROM features
) g;
About the short UPDATE syntax:
SQL update fields of one table from fields of another one
Dynamic function
Building on the simpler query, here is a dynamic function for any number of columns:
CREATE OR REPLACE FUNCTION scale_function_dyn()
RETURNS void AS
$func$
DECLARE
cols text; -- list of target columns
vals text; -- list of values to insert
aggs text; -- column list for aggregate query
BEGIN
SELECT INTO cols, vals, aggs
string_agg(quote_ident(attname), ', ')
, string_agg(format('(%I - g.%I) / g.%I'
, attname, 'avg_' || attname, 'range_' || attname), ', ')
, string_agg(format('avg(%1$I) AS %2$I, max(%1$I) - min(%1$I) AS %3$I'
, attname, 'avg_' || attname, 'range_' || attname), ', ')
FROM pg_attribute
WHERE attrelid = 'features'::regclass
AND attname NOT IN ('n', 'x0') -- exclude columns from update
AND NOT attisdropped -- no dropped (dead) columns
AND attnum > 0; -- no system columns
EXECUTE format('UPDATE features
SET (%s) = (%s)
FROM (SELECT %s FROM features) g'
, cols, vals, aggs);
END
$func$ LANGUAGE plpgsql;
Related answer with more explanation:
Update multiple columns in a trigger function in plpgsql
SQL Fiddle.