Question

My PostgreSQL database contains a table to store instances of a registered entity. This table is populated via spreadsheet upload. A web interface allows an operator to modify the information presented. However, the original data is not modified. All changes are stored in a separate table changes with the columns unique_id, column_name, value and updated_at.

Once changes are made, they are presented to the operator by first querying the original table and then querying the change table (using instance ID and the latest change date, grouped by column name). The two results are merged in PHP and presented on the web interface. This is a rather rigid way of going about the task and I would like to keep all logic within SQL.

I can easily select the latest changes for the table using the following query:

SELECT fltr_chg.unique_id, fltr_chg.column_name, chg_val.value 
FROM changes AS chg_val
JOIN ( 
      SELECT chg_rec.unique_id, chg_rec.column_name, MAX( chg_rec.updated_at )
      FROM information_schema.columns AS source
      JOIN changes AS chg_rec ON source.table_name = 'instances'
                             AND source.column_name = chg_rec.column_name
      GROUP BY chg_rec.unique_id, chg_rec.column_name
     ) AS fltr_chg ON fltr_chg.unique_id = chg_val.unique_id
                  AND fltr_chg.column_name = chg_val.column_name;

And selecting the entries from the instances table is just as easy:

SELECT * FROM instances;

Now, if there was only a way of transforming the former result and substituting the resulting values into the latter, based on the unique_id and column_name, and still retaining the result as a table, the problem would be solved. Is this possible to do?

I am sure that this is not the rarest of the problems and most likely, some systems do keep track of changes to the data in a similar way. How do they apply them back to the data if not through one of the the above described ways (current and sought solutions)?

Answer 1

Assuming Postgres 9.1 or later.
I simplified / optimized your basic query to retrieve the latest values:

SELECT DISTINCT ON (1,2)
       c.unique_id, a.attname AS col, c.value
FROM   pg_attribute a
LEFT   JOIN changes c ON c.column_name = a.attname
                     AND c.table_name  = 'instances'
                 --  AND c.unique_id   = 3  -- uncomment to fetch single row
WHERE  a.attrelid = 'instances'::regclass   -- schema-qualify to be clear?
AND    a.attnum > 0                         -- no system columns
AND    NOT a.attisdropped                   -- no deleted columns
ORDER  BY 1, 2, c.updated_at DESC;

I query the PostgreSQL catalog instead of the standard information schema because that is faster. Note the special cast to ::regclass.

Now, that gives you a table. You want all values for one unique_id in a row.
To achieve that you have basically three options:

1. One subselect (or join) per column. Expensive and unwieldy. But a valid option for only a few columns.

2. A big CASE statement.

3. A pivot function. PostgreSQL provides the crosstab() function in the additional module tablefunc for that.
   Basic instructions:

   • PostgreSQL Crosstab Query

Basic pivot table with crosstab()

I completely rewrote the function:

SELECT *
FROM   crosstab(
    $x$
    SELECT DISTINCT ON (1, 2)
           unique_id, column_name, value
    FROM   changes
    WHERE  table_name = 'instances'
 -- AND    unique_id = 3  -- un-comment to fetch single row
    ORDER  BY 1, 2, updated_at DESC;
    $x$,

    $y$
    SELECT attname
    FROM   pg_catalog.pg_attribute
    WHERE  attrelid = 'instances'::regclass  -- possibly schema-qualify table name
    AND    attnum > 0
    AND    NOT attisdropped
    AND    attname <> 'unique_id'
    ORDER  BY attnum
    $y$
    )
AS tbl (
 unique_id integer
-- !!! You have to list all columns in order here !!! --
);

I separated the catalog lookup from the value query, as the crosstab() function with two parameters provides column names separately. Missing values (no entry in changes) are substituted with NULL automatically. A perfect match for this use case!

Assuming that attname matches column_name. Excluding unique_id, which plays a special role.

Full automation

Addressing your comment: There is a way to supply the column definition list automatically. It’s not for the faint of heart, though.

I use a number of advanced Postgres features here: crosstab(), plpgsql function with dynamic SQL, composite type handling, advanced dollar quoting, catalog lookup, aggregate function, window function, object identifier type, …

Test environment:

CREATE TABLE instances (
  unique_id int
, col1      text
, col2      text -- two columns are enough for the demo
);

INSERT INTO instances VALUES
  (1, 'foo1', 'bar1')
, (2, 'foo2', 'bar2')
, (3, 'foo3', 'bar3')
, (4, 'foo4', 'bar4');

CREATE TABLE changes (
  unique_id   int
, table_name  text
, column_name text
, value       text
, updated_at  timestamp
);

INSERT INTO changes VALUES
  (1, 'instances', 'col1', 'foo11', '2012-04-12 00:01')
, (1, 'instances', 'col1', 'foo12', '2012-04-12 00:02')
, (1, 'instances', 'col1', 'foo1x', '2012-04-12 00:03')
, (1, 'instances', 'col2', 'bar11', '2012-04-12 00:11')
, (1, 'instances', 'col2', 'bar17', '2012-04-12 00:12')
, (1, 'instances', 'col2', 'bar1x', '2012-04-12 00:13')

, (2, 'instances', 'col1', 'foo2x', '2012-04-12 00:01')
, (2, 'instances', 'col2', 'bar2x', '2012-04-12 00:13')

 -- NO change for col1 of row 3 - to test NULLs
, (3, 'instances', 'col2', 'bar3x', '2012-04-12 00:13');

 -- NO changes at all for row 4 - to test NULLs

Automated function for one table

CREATE OR REPLACE FUNCTION f_curr_instance(int, OUT t public.instances) AS
$func$
BEGIN
   EXECUTE $f$
   SELECT *
   FROM   crosstab($x$
      SELECT DISTINCT ON (1,2)
             unique_id, column_name, value
      FROM   changes
      WHERE  table_name = 'instances'
      AND    unique_id =  $f$ || $1 || $f$
      ORDER  BY 1, 2, updated_at DESC;
      $x$
    , $y$
      SELECT attname
      FROM   pg_catalog.pg_attribute
      WHERE  attrelid = 'public.instances'::regclass
      AND    attnum > 0
      AND    NOT attisdropped
      AND    attname <> 'unique_id'
      ORDER  BY attnum
      $y$) AS tbl ($f$
   || (SELECT string_agg(attname || ' ' || atttypid::regtype::text
                       , ', ' ORDER BY attnum) -- must be in order
       FROM   pg_catalog.pg_attribute
       WHERE  attrelid = 'public.instances'::regclass
       AND    attnum > 0
       AND    NOT attisdropped)
   || ')'
   INTO t;
END
$func$  LANGUAGE plpgsql;

The table instances is hard-coded, schema qualified to be unambiguous. Note the use of the table type as return type. There is a row type registered automatically for every table in PostgreSQL. This is bound to match the return type of the crosstab() function.

This binds the function to the type of the table:

• You will get an error message if you try to DROP the table
• Your function will fail after an ALTER TABLE. You have to recreate it (without changes). I consider this a bug in 9.1. ALTER TABLE shouldn’t silently break the function, but raise an error.

This performs very well.

Call:

SELECT * FROM f_curr_instance(3);

unique_id | col1  | col2
----------+-------+-----
 3        |<NULL> | bar3x

Note how col1 is NULL here.
Use in a query to display an instance with its latest values:

SELECT i.unique_id
     , COALESCE(c.col1, i.col1)
     , COALESCE(c.col2, i.col2)
FROM   instances i
LEFT   JOIN f_curr_instance(3) c USING (unique_id)
WHERE  i.unique_id = 3;

Full automation for any table

(Added 2016. This is dynamite.)
Requires Postgres 9.1 or later. (Could be made out to work with pg 8.4, but I didn’t bother to backpatch.)

CREATE OR REPLACE FUNCTION f_curr_instance(_id int, INOUT _t ANYELEMENT) AS
$func$
DECLARE
   _type text := pg_typeof(_t);
BEGIN
   EXECUTE
   (
   SELECT format
         ($f$
         SELECT *
         FROM   crosstab(
            $x$
            SELECT DISTINCT ON (1,2)
                   unique_id, column_name, value
            FROM   changes
            WHERE  table_name = %1$L
            AND    unique_id  = %2$s
            ORDER  BY 1, 2, updated_at DESC;
            $x$    
          , $y$
            SELECT attname
            FROM   pg_catalog.pg_attribute
            WHERE  attrelid = %1$L::regclass
            AND    attnum > 0
            AND    NOT attisdropped
            AND    attname <> 'unique_id'
            ORDER  BY attnum
            $y$) AS ct (%3$s)
         $f$
          , _type, _id
          , string_agg(attname || ' ' || atttypid::regtype::text
                     , ', ' ORDER BY attnum)  -- must be in order
         )
   FROM   pg_catalog.pg_attribute
   WHERE  attrelid = _type::regclass
   AND    attnum > 0
   AND    NOT attisdropped
   )
   INTO _t;
END
$func$  LANGUAGE plpgsql;

Call (providing the table type with NULL::public.instances:

SELECT * FROM f_curr_instance(3, NULL::public.instances);

Related:

• Refactor a PL/pgSQL function to return the output of various SELECT queries
• How to set value of composite variable field using dynamic SQL