Casting NULL type when updating multiple rows

If you have a script generating the query you could extract and cache the data type of each column an create the type cast accordingly. E.g:

SELECT column_name,data_type,udt_name 
FROM information_schema.columns 
WHERE table_name = 'foo';

From this udt_name you'll get the necessary cast as you explained in the last paragraph. Additionally you could do this:

UPDATE foo
SET x = t.x
FROM (VALUES(null::int4,756),(null::int4,6300))
AS t(x,pkid)
WHERE foo.pkid = t.pkid;

With a standalone VALUES expression PostgreSQL has no idea what the data types should be. With simple numeric literals the system is happy to assume matching types. But with other input (like NULL) you would need to cast explicitly - as you already have found out.

You can query pg_catalog (fast, but PostgreSQL-specific) or the information_schema (slow, but standard SQL) to find out and prepare your statement with appropriate types.

Or you can use one of these simple "tricks" (I saved the best for last):

0. Select row with LIMIT 0, append rows with UNION ALL VALUES

UPDATE foo f
SET    x = t.x
     , y = t.y
FROM  (
  (SELECT pkid, x, y FROM foo LIMIT 0) -- parenthesis needed with LIMIT
   UNION ALL
   VALUES
      (1, 20, NULL)  -- no type casts here
    , (2, 50, NULL)
   ) t               -- column names and types are already defined
WHERE  f.pkid = t.pkid;

The first sub-select of the subquery:

(SELECT x, y, pkid  FROM foo LIMIT 0)

gets names and types for the columns, but LIMIT 0 prevents it from adding an actual row. Subsequent rows are coerced to the now well-defined row type - and checked immediately whether they match the type. Should be a subtle additional improvement over your original form.

While providing values for all columns of the table this short syntax can be used for the first row:

(TABLE foo LIMIT 0)

Major limitation: Postgres casts the input literals of the free-standing VALUES expression to a "best-effort" type immediately. When it later tries to cast to the given types of the first SELECT, it may already be too late for some types if there is no registered assignment cast between the assumed type and the target type. Examples: text -> timestamp or text -> json.

Pro:

  • Minimum overhead.
  • Readable, simple and fast.
  • You only need to know relevant column names of the table.

Con:

  • Type resolution can fail for some types.

1. Select row with LIMIT 0, append rows with UNION ALL SELECT

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

Pro:

  • Like 0., but avoids failing type resolution.

Con:

  • UNION ALL SELECT is slower than VALUES expression for long lists of rows, as you found in your test.
  • Verbose syntax per row.

2. VALUES expression with per-column type

...
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.
...

Contrary to 0. this avoids premature type resolution.

The first row in the VALUES expression is a row of NULL values which defines the type for all subsequent rows. This leading noise row is filtered by WHERE f.pkid = t.pkid later, so it never sees the light of day. For other purposes you can eliminate the added first row with OFFSET 1 in a subquery.

Pro:

  • Typically faster than 1. (or even 0.)
  • Short syntax for tables with many columns and only few are relevant.
  • You only need to know relevant column names of the table.

Con:

  • Verbose syntax for only few rows
  • Less readable (IMO).

3. VALUES expression with row type

UPDATE foo f
SET x = (t.r).x         -- parenthesis needed to make syntax unambiguous
  , y = (t.r).y
FROM (
   VALUES
      ('(1,20,)'::foo)  -- columns need to be in default order of table
     ,('(2,50,)')       -- nothing after the last comma for NULL
   ) t (r)              -- column name for row type
WHERE  f.pkid = (t.r).pkid;

You obviously know the table name. If you also know the number of columns and their order you can work with this.

For every table in PostgreSQL a row type is registered automatically. If you match the number of columns in your expression, you can cast to the row type of the table ('(1,50,)'::foo) thereby assigning column types implicitly. Put nothing behind a comma to enter a NULL value. Add a comma for every irrelevant trailing column.
In the next step you can access individual columns with the demonstrated syntax. More about Field Selection in the manual.

Or you could add a row of NULL values and use uniform syntax for actual data:

...
  VALUES
      ((NULL::foo))  -- row of NULL values
    , ('(1,20,)')    -- uniform ROW value syntax for all
    , ('(2,50,)')
...

Pro:

  • Fastest (at least in my tests with few rows and columns).
  • Shortest syntax for few rows or tables where you need all columns.
  • You don't have to spell out columns of the table - all columns automatically have the matching name.

Con:

  • Not so well known syntax for field selection from record / row / composite type.
  • You need to know number and position of relevant columns in default order.

4. VALUES expression with decomposed row type

Like 3., but with decomposed rows in standard syntax:

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 (I made them match)
WHERE  f.pkid = t.pkid;     -- eliminates 1st row with NULL values

Or, with a leading row of NULL values again:

...
   VALUES
      ((NULL::foo).*)  -- row of NULL values
    , (1, 20, NULL)    -- uniform syntax for all
    , (2, 50, NULL)
...

Pros and cons like 3., but with more commonly known syntax.
And you need to spell out column names (if you need them).

5. VALUES expression with types fetched from row type

Like Unril commented, we can combine the virtues of 2. and 4. to provide only a subset of columns:

UPDATE foo f
SET   (  x,   y)
    = (t.x, t.y)  -- short notation, see below
FROM (
   VALUES
      ((NULL::foo).pkid, (NULL::foo).x, (NULL::foo).y)  -- subset of columns
    , (1, 20, NULL)
    , (2, 50, NULL)
   ) t(pkid, x, y)       -- arbitrary column names (I made them match)
WHERE  f.pkid = t.pkid;

Pros and cons like 4., but we can work with any subset of columns and don't have to know the full list.

Also displaying short syntax for the UPDATE itself that's convenient for cases with many columns. Related:

  • Bulk update of all columns

4. and 5. are my favorites.

db<>fiddle here - demonstrating all

Tags:

Sql

Types

Postgresql

Sql Update

Casting

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