Handle result when dynamic SQL is in a loop

问题

I have a bunch of table that have a "stat" column (stat for status ;-)

I would like the count of each stats, and see it!

My tables look like this

create table a (
   a_id UUID PRIMARY KEY DEFAULT uuid_generate_v4(),
   a_stat status_t
);
create table b (
   b_id UUID PRIMARY KEY DEFAULT uuid_generate_v4(),
   b_stat status_t
);

status_t is an enum.

So I did this:

DO $$
DECLARE
  tableName RECORD;
  result RECORD;
BEGIN
  SET SEARCH_PATH = projet, public;

  FOR tableName IN SELECT
             c.relname,
             a.attname
           FROM pg_class AS c
             INNER JOIN pg_attribute AS a ON a.attrelid = c.oid
           WHERE a.attname LIKE '%stat' AND c.relkind = 'r' LOOP

      EXECUTE format('SELECT %I, count(%I) FROM %I GROUP BY %I',
                     tableName.attname, tableName.attname, tableName.relname, tableName.attname) INTO result;
      SELECT * FROM result;
  END LOOP;
END;
$$;

There are things that I think I'm not doing well here.

There is maybe a better form of format
I can't select a RECORD, I think the datatype is not good (but can't figure out what type I should use)
A select inside a for loop is not a good idea (I think ?) But I didn't found How to put result into a array of result, and display it after the for loop.

How to do this properly?

回答1:

You cannot return from a DO command. You could raise notices or write to a temporary table to work around this. But rather use a proper function instead. Like this:

CREATE OR REPLACE FUNCTION foo()
  RETURNS TABLE (sch_name text, tbl_name text, col_name text, row_count_notnull int8) AS
$func$
DECLARE
   _sch text;
   _tbl text;
   _col text;
BEGIN
  FOR _sch, _tbl, _col IN
      SELECT c.relnamespace::regnamespace, c.relname, a.attname
      FROM   pg_class     c
      JOIN   pg_attribute a ON a.attrelid = c.oid
      WHERE  c.relnamespace = ANY ('{projet, public}'::regnamespace[])  -- project?
      AND    c.relkind = 'r'           -- only regular tables
      AND    a.attname LIKE '%\_stat'  -- a_stat, b_stat
      AND    a.attnum > 0              -- only user columns (redundant here)
      AND    NOT a.attisdropped        -- exclude dropped columns
   LOOP
      RETURN QUERY EXECUTE format(
         'SELECT $1, $2, $3, count(%I) FROM %I.%I GROUP BY 1'
       , _col, _sch, _tbl)
      USING _sch, _tbl, _col;
   END LOOP;
END
$func$  LANGUAGE plpgsql;

Call:

SELECT * FROM foo();

Major points:

You cannot not SELECT without target (~~SELECT * FROM result;~~), neither in a DO command nor in a plpgsql function. (You could in a plain SQL function, but you have no loop there.) I return results with RETURN QUERY EXECUTE.
Pass values to EXECUTE with the USING clause.
Schema-qualify tables in the dynamic query. Else, you might query the wrong table by accident.
Does not include the temporary schema like your attempt (even if you were not aware of it). You could add it using pg_my_temp_schema() if you want, but you probably did not want it anyway:
- How does the search_path influence identifier resolution and the "current schema"
- Temporary schema per connection?

Table name as a PostgreSQL function parameter

Search for more examples with dynamic SQL in plpgsql here on SO.

来源：https://stackoverflow.com/questions/46282882/handle-result-when-dynamic-sql-is-in-a-loop

标签

postgresql

count

plpgsql

dynamic-sql