Simulate CREATE DATABASE IF NOT EXISTS for PostgreSQL?

Question

I want to create a database which does not exist through JDBC. Unlike MySQL, PostgreSQL does not support create if not exists syntax. What is the best way to accomplish this?

The application does not know if the database exists or not. It should check and if the database exists it should be used. So it makes sense to connect to the desired database and if connection fails due to non-existence of database it should create new database (by connecting to the default postgres database). I checked the error code returned by Postgres but I could not find any relevant code that species the same.

Another method to achieve this would be to connect to the postgres database and check if the desired database exists and take action accordingly. The second one is a bit tedious to work out.

Is there any way to achieve this functionality in Postgres?

Solution 1

Restrictions

You can ask the system catalog pg_database - accessible from any database in the same database cluster. The tricky part is that CREATE DATABASE can only be executed as a single statement. The manual:

CREATE DATABASE cannot be executed inside a transaction block.

So it cannot be run directly inside a function or DO statement, where it would be inside a transaction block implicitly. SQL procedures, introduced with Postgres 11, cannot help with this either.

Workaround from within psql

You can work around it from within psql by executing the DDL statement conditionally:

SELECT 'CREATE DATABASE mydb'
WHERE NOT EXISTS (SELECT FROM pg_database WHERE datname = 'mydb')\gexec

The manual:

\gexec

Sends the current query buffer to the server, then treats each column of each row of the query's output (if any) as a SQL statement to be executed.

Workaround from the shell

With \gexec you only need to call psql once:

echo "SELECT 'CREATE DATABASE mydb' WHERE NOT EXISTS (SELECT FROM pg_database WHERE datname = 'mydb')\gexec" | psql

You may need more psql options for your connection; role, port, password, ... See:

• Run batch file with psql command without password

The same cannot be called with psql -c "SELECT ...\gexec" since \gexec is a psql meta‑command and the -c option expects a single command for which the manual states:

command must be either a command string that is completely parsable by the server (i.e., it contains no psql-specific features), or a single backslash command. Thus you cannot mix SQL and psql meta-commands within a -c option.

Workaround from within Postgres transaction

You could use a dblink connection back to the current database, which runs outside of the transaction block. Effects can therefore also not be rolled back.

Install the additional module dblink for this (once per database):

• How to use (install) dblink in PostgreSQL?

Then:

DO
$do$
BEGIN
   IF EXISTS (SELECT FROM pg_database WHERE datname = 'mydb') THEN
      RAISE NOTICE 'Database already exists';  -- optional
   ELSE
      PERFORM dblink_exec('dbname=' || current_database()  -- current db
                        , 'CREATE DATABASE mydb');
   END IF;
END
$do$;

Again, you may need more psql options for the connection. See Ortwin's added answer:

• Simulate CREATE DATABASE IF NOT EXISTS for PostgreSQL?

Detailed explanation for dblink:

• How do I do large non-blocking updates in PostgreSQL?

You can make this a function for repeated use.

Solution 2

another alternative, just in case you want to have a shell script which creates the database if it does not exist and otherwise just keeps it as it is:

psql -U postgres -tc "SELECT 1 FROM pg_database WHERE datname = 'my_db'" | grep -q 1 || psql -U postgres -c "CREATE DATABASE my_db"

I found this to be helpful in devops provisioning scripts, which you might want to run multiple times over the same instance.

For those of you who would like an explanation:

-c = run command in database session, command is given in string
-t = skip header and footer
-q = silent mode for grep 
|| = logical OR, if grep fails to find match run the subsequent command

Solution 3

If you don't care about the data, you can drop database first and then recreate it:

DROP DATABASE IF EXISTS dbname;
CREATE DATABASE dbname;

Solution 4

PostgreSQL does not support IF NOT EXISTS for CREATE DATABASE statement. It is supported only in CREATE SCHEMA. Moreover CREATE DATABASE cannot be issued in transaction therefore it cannot be in DO block with exception catching.

When CREATE SCHEMA IF NOT EXISTS is issued and schema already exists then notice (not error) with duplicate object information is raised.

To solve these problems you need to use dblink extension which opens a new connection to database server and execute query without entering into transaction. You can reuse connection parameters with supplying empty string.

Below is PL/pgSQL code which fully simulates CREATE DATABASE IF NOT EXISTS with same behavior like in CREATE SCHEMA IF NOT EXISTS. It calls CREATE DATABASE via dblink, catch duplicate_database exception (which is issued when database already exists) and converts it into notice with propagating errcode. String message has appended , skipping in the same way how it does CREATE SCHEMA IF NOT EXISTS.

CREATE EXTENSION IF NOT EXISTS dblink;

DO $$
BEGIN
PERFORM dblink_exec('', 'CREATE DATABASE testdb');
EXCEPTION WHEN duplicate_database THEN RAISE NOTICE '%, skipping', SQLERRM USING ERRCODE = SQLSTATE;
END
$$;

This solution is without any race condition like in other answers, where database can be created by external process (or other instance of same script) between checking if database exists and its own creation.

Moreover when CREATE DATABASE fails with other error than database already exists then this error is propagated as error and not silently discarded. There is only catch for duplicate_database error. So it really behaves as IF NOT EXISTS should.

You can put this code into own function, call it directly or from transaction. Just rollback (restore dropped database) would not work.

Testing output (called two times via DO and then directly):

$ sudo -u postgres psql
psql (9.6.12)
Type "help" for help.

postgres=# \set ON_ERROR_STOP on
postgres=# \set VERBOSITY verbose
postgres=# 
postgres=# CREATE EXTENSION IF NOT EXISTS dblink;
CREATE EXTENSION
postgres=# DO $$
postgres$# BEGIN
postgres$# PERFORM dblink_exec('', 'CREATE DATABASE testdb');
postgres$# EXCEPTION WHEN duplicate_database THEN RAISE NOTICE '%, skipping', SQLERRM USING ERRCODE = SQLSTATE;
postgres$# END
postgres$# $$;
DO
postgres=# 
postgres=# CREATE EXTENSION IF NOT EXISTS dblink;
NOTICE:  42710: extension "dblink" already exists, skipping
LOCATION:  CreateExtension, extension.c:1539
CREATE EXTENSION
postgres=# DO $$
postgres$# BEGIN
postgres$# PERFORM dblink_exec('', 'CREATE DATABASE testdb');
postgres$# EXCEPTION WHEN duplicate_database THEN RAISE NOTICE '%, skipping', SQLERRM USING ERRCODE = SQLSTATE;
postgres$# END
postgres$# $$;
NOTICE:  42P04: database "testdb" already exists, skipping
LOCATION:  exec_stmt_raise, pl_exec.c:3165
DO
postgres=# 
postgres=# CREATE DATABASE testdb;
ERROR:  42P04: database "testdb" already exists
LOCATION:  createdb, dbcommands.c:467

Solution 5

I had to use a slightly extended version @Erwin Brandstetter used:

DO
$do$
DECLARE
  _db TEXT := 'some_db';
  _user TEXT := 'postgres_user';
  _password TEXT := 'password';
BEGIN
  CREATE EXTENSION IF NOT EXISTS dblink; -- enable extension 
  IF EXISTS (SELECT 1 FROM pg_database WHERE datname = _db) THEN
    RAISE NOTICE 'Database already exists';
  ELSE
    PERFORM dblink_connect('host=localhost user=' || _user || ' password=' || _password || ' dbname=' || current_database());
    PERFORM dblink_exec('CREATE DATABASE ' || _db);
  END IF;
END
$do$

I had to enable the dblink extension, plus i had to provide the credentials for dblink. Works with Postgres 9.4.

View more solutions

Comments

[Anton Anikeev]

It doesn't works for me. c:\Program Files\PostgreSQL\9.6\bin $ psql.exe -U admin -tc "SELECT 1 FROM pg_database WHERE datname = 'my_db'" | grep -q 1 || psql -U admin -c "CREATE DATABASE my_db" 'grep' is not recognized as an internal or external command, operable program or batch file. What did I do wrong ?

[thulashi]

You don't have grep in your path. On Windows, grep is not installed by default. You can search for gnu grep windows to find a version that could work on Windows.

[Anton Anikeev]

Thx @Rod. After I installed grep this script worked for me.

[Ondrej Burkert]

I ran into problem with this when creating a database on AWS RDS Postgres from remote. RDS master user is not a super user and is hence not allowed to use dblink_connect.

[Erwin Brandstetter]

If you don't have superuser privileges, you can use a password for the connection. Details: dba.stackexchange.com/a/105186/3684

[a_horse_with_no_name]

There is no if not exists for CREATE DATABASE - not even in Postgres 11 postgresql.org/docs/current/static/sql-createdatabase.html

[Micheal J. Roberts]

Worked like a charm, used within an init.sql script inside Docker container. Thanks!

[Erwin Brandstetter]

@AntonAnikeev: Can be done with a single psql call without grep. I added solutions to my answer.

[FilBot3]

I had to drop the \gexec when I ran the first query from the shell, but it worked.

[vog]

This is currently the only correct answer here, which does not suffer from race conditions, and uses the necessary selective error handling. It is a realy pity that this answer appeared after the (not fully correct) top answer collected more that 70 points.

[Oliver]

I find it useful to first us pg_isready to check that a connection is possible; if a connection is not available (wrong hostname, network down etc), the script will attempt to create the database and will fail with possibly confusing error message

[András Aszódi]

Very elegant solution. Just don't forget to back up the database first if you do care about the data. For testing situations though this is my preferred solution.

[Pali]

Well, other answers are not such precise to handle all possible corner cases which may happen. You can also call my PL/pgSQL code more times in parallel and it does not fail.

[Nux]

Note. You might want to add -X and -A options to first psql to get more consistent results. You could also then just write the value to a variable and use more readable if statement dbExists=`psql -U postgres -X -A -t -c ...` and if [ -z "$dbExists" ]; then... else... fi.

[Reda Drissi]

OP doesn't want something that sends an error, because it might crash a script.

[prembhaskal]

even this answer can suffer from race conditions as explained here. stackoverflow.com/a/63106063/1866530

[Jean F.]

script.sh &> /dev/null so it won't crash

[emi]

Database creation may crash on other conditions than already existence. The other causes will be silently hidden with your solution.