How to Handle NULL Type Casting in PostgreSQL Multi-Row Updates?

Casting NULL Type in Multi-Row Updates

In PostgreSQL, executing an update query on multiple rows can lead to errors when dealing with NULL values if the column types are not explicitly cast. This article explores several solutions to this issue, providing alternative approaches to ensure proper type casting during multi-row updates.

Solution 1: Select Limit 0 with VALUES and UNION ALL

UPDATE foo f
SET    x = t.x
     , y = t.y
FROM  (
  (SELECT pkid, x, y FROM foo LIMIT 0) -- Get column types
   UNION ALL
   VALUES
      (1, 20, NULL)  -- No type casts
    , (2, 50, NULL)
   ) t               -- Column names and types defined
WHERE  f.pkid = t.pkid;

This method combines a SELECT statement with a LIMIT of 0 to retrieve column names and types and then appends the desired data rows using the UNION ALL operator. The first row of the subquery ensures that the appropriate column types are defined for the subsequent rows.

Solution 2: Select Limit 0 with VALUES and UNION ALL SELECT

UPDATE foo f
SET    x = t.x
     , y = t.y
FROM  (
  (SELECT pkid, x, y FROM foo LIMIT 0) -- Get column types
   UNION ALL SELECT 1, 20, NULL
   UNION ALL SELECT 2, 50, NULL
   ) t               -- Column names and types defined
WHERE  f.pkid = t.pkid;

Similar to Solution 1, this approach uses SELECT to get column types and then uses individual SELECT statements to append data rows, preventing any premature type casting.

Solution 3: VALUES Expression with Per-Column Type

UPDATE foo f
SET    x = t.x
     , y = t.y
FROM  (
   VALUES 
     ((SELECT pkid FROM foo LIMIT 0)
    , (SELECT x    FROM foo LIMIT 0)
    , (SELECT y    FROM foo LIMIT 0))  -- Get type for each col individually
   , (1, 20, NULL)
   , (2, 50, NULL)
   ) t (pkid, x, y)  -- Columns names not defined yet, only types.
...

This solution defines the column types within the VALUES expression itself, ensuring that the subsequent rows are cast to those types without encountering any errors due to automatic type assumptions.

Solution 4: VALUES Expression with Row Type

UPDATE foo f
SET x = (t.r).x         -- Parenthesis for unambiguous syntax
  , y = (t.r).y
FROM (
   VALUES
      ('(1,20,)'::foo)  -- Columns need to be in table default order
     ,('(2,50,)')       -- Nothing after last comma for NULL
   ) t (r)              -- Column name for row type
WHERE  f.pkid = (t.r).pkid;

This approach uses the row type of the specific table, allowing you to cast columns to the correct types implicitly. You can access individual column values using field selection syntax.

Solution 5: VALUES Expression with Decomposed Row Type

UPDATE foo f
SET    x = t.x
     , y = t.y
FROM (
   VALUES
      (('(1,20,)'::foo).*)  -- Decomposed row of values
    , (2, 50, NULL)
   ) t(pkid, x, y)  -- Arbitrary column names (match table columns)
WHERE  f.pkid = t.pkid;     -- Eliminates 1st row with NULL values

Similar to Solution 4, but using decomposed rows to specify data values. This allows you to provide only the relevant columns, eliminating the need to know the complete order and types of all columns in the table.

Choosing the best solution depends on factors such as performance, convenience, and the availability of information about column types.

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