Behaviour of NOT LIKE with NULL values - sql

Question

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.

Answer 1

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:
How does the search_path influence identifier resolution and the "current schema"
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:
Auto increment table column
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:
Find the referenced table name using table, field and schema name
Get the default values of table columns in Postgres?

Answer 2

I think you can use :
SELECT column_name *FROM* information_schema.columns
WHERE table_name = 'table1'
AND ( nvl(column_default,0) *NOT LIKE* 'nextval%');