sqlpostgresqlnullinformation-schema

Behaviour of NOT LIKE with NULL values

I want to fetch all columns of a table except of columns of type serial. The closest query to this problem I was able to come up with this one:

SELECT column_name FROM information_schema.columns
WHERE table_name = 'table1' AND column_default NOT LIKE 'nextval%'

But the problem is its also excluding/filtering rows having empty values for column_default.I don't know why the behaviour of Postgres is like this. So I had to change my query to something like this:

SELECT column_name FROM information_schema.columns
WHERE table_name = 'table1'
AND ( column_default IS NULL OR column_default NOT LIKE 'nextval%')

Any better suggestions or rationale behind this are welcome.

Solution

  • About null

    'anything' NOT LIKE null yields null, not true.
    And only true qualifies for filter expressions in a WHERE clause.

    Most functions return null on null input (there are exceptions). That's the nature of null in any proper RDBMS.

    If you desire a single expression, you could use:

    AND (column_default LIKE 'nextval%') IS NOT TRUE;

    That's hardly shorter or faster, though. Details in the manual.

    Proper query

    Your query is still unreliable. A table name alone is not unique in a Postgres database, you need to specify the schema name in addition or rely on the current search_path to find the first match in it:

    Related:

    SELECT column_name
FROM   information_schema.columns
WHERE  table_name = 'hstore1'
AND    table_schema = 'public'   -- your schema!
AND   (column_default IS NULL
    OR column_default NOT LIKE 'nextval%');

    Better, but still not bullet-proof. A column default starting with 'nextval' does not make a serial, yet. See:

    To be sure, check whether the sequence in use is "owned" by the column with pg_get_serial_sequence(table_name, column_name).

    I rarely use the information schema myself. Those slow, bloated views guarantee portability across major versions - and aim at portability to other standard-compliant RDBMS. But too much is incompatible anyway. Oracle does not even implement the information schema (as of 2015).

    Also, useful Postgres-specific columns are missing in the information schema. For this case I might query the the system catalogs like this:

    SELECT *
FROM   pg_catalog.pg_attribute a
WHERE  attrelid = 'table1'::regclass
AND    NOT attisdropped   -- no dropped (dead) columns
AND    attnum > 0         -- no system columns
AND    NOT EXISTS (
   SELECT FROM pg_catalog.pg_attrdef d
   WHERE  (d.adrelid, d.adnum) = (a.attrelid, a.attnum)
   AND    d.adsrc LIKE 'nextval%'
   AND    pg_get_serial_sequence(a.attrelid::regclass::text, a.attname) <> ''
   );

    Faster and more reliable, but less portable.

    The manual:

    The catalog pg_attrdef stores column default values. The main information about columns is stored in pg_attribute (see below). Only columns that explicitly specify a default value (when the table is created or the column is added) will have an entry here.

    'table1'::regclass uses the search_path to resolve the name, which avoids ambiguity. You can schema-qualify the name to overrule: 'myschema.table1'::regclass.

    Related: