调试PL / pgSQL函数

Question

I am trying to get this PL/pgSQL function to work :

CREATE OR REPLACE FUNCTION loopcolumns2(tableName TEXT, pourcentage real)
RETURNS void AS $$
DECLARE 
    _name text; 
    missing_percentage real;
BEGIN
    FOR _name IN SELECT column_name from information_schema.columns where table_name=tableName LOOP
        SELECT 100 - (count(_name) * 100) / count(*) INTO missing_percentage FROM tableName;
        IF (missing_percentage > pourcentage)
            THEN ALTER TABLE tableName DROP COLUMN _name;
        END IF;
    END LOOP;
END; $$ LANGUAGE plpgsql;

The goal of the function is to loop through all the columns of a table, a delete the columns where the percentage of missing values is bigger than an input percentage.

I get the following error :

SELECT 100 - (count( $1 ) * 100) / count(*) FROM  $2
                                                  ^
CONTEXT:  SQL statement in PL/PgSQL function "loopcolumns2" near line 6

Answer 1

You're trying to SELECT from text stored by tableName vaiable. You cannot do that because Postgres think you want him to select from table named tableName , but probably such table doesn't exist.

You nead to create dynamic query in string and use EXECUTE ... INTO ... statement . Like this:

DECLARE query TEXT;
...

query :=  'SELECT 100 - (count(' || _name::TEXT || ') * 100) / count(*) FROM '
          || tableName::TEXT;

EXECUTE query INTO percentage ;
...