How to check if a table exists in a given schema
Postgres 8.4 and greater databases contain common tables in public
schema and company specific tables in company
schema.company
schema names always start with 'company'
and end with the company number.
So there may be schemas like:
public
company1
company2
company3
...
companynn
An application always works with a single company.
The search_path
is specified accordingly in odbc or npgsql connection string, like:
search_path='company3,public'
How would you check if a given table exists in a specified companyn
schema?
eg:
select isSpecific('company3','tablenotincompany3schema')
should return false
, and
select isSpecific('company3','tableincompany3schema')
should return true
.
In any case, the function should check only companyn
schema passed, not other schemas.
If a given table exists in both public
and the passed schema, the function should return true
.
It should work for Postgres 8.4 or later.
Solution 1:
It depends on what you want to test exactly.
Information schema?
To find "whether the table exists" (no matter who's asking), querying the information schema (information_schema.tables
) is incorrect, strictly speaking, because (per documentation):
Only those tables and views are shown that the current user has access to (by way of being the owner or having some privilege).
The query provided by @kong can return FALSE
, but the table can still exist. It answers the question:
How to check whether a table (or view) exists, and the current user has access to it?
SELECT EXISTS (
SELECT FROM information_schema.tables
WHERE table_schema = 'schema_name'
AND table_name = 'table_name'
);
The information schema is mainly useful to stay portable across major versions and across different RDBMS. But the implementation is slow, because Postgres has to use sophisticated views to comply to the standard (information_schema.tables
is a rather simple example). And some information (like OIDs) gets lost in translation from the system catalogs - which actually carry all information.
System catalogs
Your question was:
How to check whether a table exists?
SELECT EXISTS (
SELECT FROM pg_catalog.pg_class c
JOIN pg_catalog.pg_namespace n ON n.oid = c.relnamespace
WHERE n.nspname = 'schema_name'
AND c.relname = 'table_name'
AND c.relkind = 'r' -- only tables
);
Use the system catalogs pg_class
and pg_namespace
directly, which is also considerably faster. However, per documentation on pg_class
:
The catalog
pg_class
catalogs tables and most everything else that has columns or is otherwise similar to a table. This includes indexes (but see alsopg_index
), sequences, views, materialized views, composite types, and TOAST tables;
For this particular question you can also use the system view pg_tables
. A bit simpler and more portable across major Postgres versions (which is hardly of concern for this basic query):
SELECT EXISTS (
SELECT FROM pg_tables
WHERE schemaname = 'schema_name'
AND tablename = 'table_name'
);
Identifiers have to be unique among all objects mentioned above. If you want to ask:
How to check whether a name for a table or similar object in a given schema is taken?
SELECT EXISTS (
SELECT FROM pg_catalog.pg_class c
JOIN pg_catalog.pg_namespace n ON n.oid = c.relnamespace
WHERE n.nspname = 'schema_name'
AND c.relname = 'table_name'
);
- Related answer on dba.SE discussing "Information schema vs. system catalogs"
Alternative: cast to regclass
SELECT 'schema_name.table_name'::regclass
This raises an exception if the (optionally schema-qualified) table (or other object occupying that name) does not exist.
If you do not schema-qualify the table name, a cast to regclass
defaults to the search_path
and returns the OID for the first table found - or an exception if the table is in none of the listed schemas. Note that the system schemas pg_catalog
and pg_temp
(the schema for temporary objects of the current session) are automatically part of the search_path
.
You can use that and catch a possible exception in a function. Example:
- Check if sequence exists in Postgres (plpgsql)
A query like above avoids possible exceptions and is therefore slightly faster.
to_regclass(rel_name)
in Postgres 9.4+
Much simpler now:
SELECT to_regclass('schema_name.table_name');
Same as the cast, but it returns ...
... null rather than throwing an error if the name is not found
Solution 2:
Perhaps use information_schema:
SELECT EXISTS(
SELECT *
FROM information_schema.tables
WHERE
table_schema = 'company3' AND
table_name = 'tableincompany3schema'
);