Connect to Databases

This document describes how to connect to SynxDB.

Connection parameters

You can connect to SynxDB using a PostgreSQL-compatible client program, such as psql. Users and administrators always connect to SynxDB through the coordinator. Segments do not accept client connections.

To establish a connection to the SynxDB coordinator, you need to know the following connection information and configure your client program accordingly.

Connection parameter	Description	Environment variable
Application name	The name of the application connecting to the database. The default value for the `application_name` connection parameter is psql.	`$PGAPPNAME`
Database name	The name of the database to connect to. For a newly initialized system, use the `postgres` database for the first connection.	`$PGDATABASE`
Host name	The host name of the SynxDB coordinator. The default is localhost.	`$PGHOST`
Port	The port number on which the SynxDB coordinator instance is running. The default value is `5432`.	`$PGPORT`
User name	The database user (role) name for the connection. This is not necessarily the same as your operating system user name. If you are unsure of your database user name, contact your SynxDB administrator. Note that each SynxDB system automatically creates a superuser account upon initialization. The name of this account is the same as the operating system user name of the user who initialized the SynxDB system (usually `gpadmin`).	`$PGUSER`

The Connect using psql section provides example commands for connecting to SynxDB.

Supported client applications

You can use various client applications to connect to SynxDB:

Some client applications are included with the SynxDB installation. Among them, psql is an interactive command-line interface.
Using standard database application interfaces, you can create your own client applications to connect with SynxDB.
Most client tools that use standard database interfaces, such as ODBC and JDBC, can be configured to connect to SynxDB.

Client applications

The SynxDB installation includes several client utility applications, located in the $GPHOME/bin directory of the coordinator host installation. The following are the most commonly used client applications:

Name	Purpose
`createdb`	Create a new database
`createuser`	Define a new database role
`dropdb`	Remove a database
`dropuser`	Remove a role
`psql`	PostgreSQL interactive terminal
`reindexdb`	Reindex a database
`vacuumdb`	Collect garbage and analyze a database

When using these client applications, you must connect to the database through the coordinator instance. You need to know the name of the target database, the host name and port number of the coordinator, and the database user name to connect with. You can specify this information using the -d, -h, -p, and -U options on the connection command, respectively. If an option is not specified for a parameter, its default value is used.

All these options have default values, which are used if an option is not specified: The default host is localhost. The default port number is 5432. The default user name is the operating system user name, and so is the database name. Note that the operating system user name and the SynxDB user name are not necessarily the same.

If the default values are not correct, you can set the PGDATABASE, PGHOST, PGPORT, and PGUSER environment variables accordingly, or use the ~/.pgpass file to store frequently used passwords.

Connect using psql

Depending on the default values or the environment variables you have set, use psql to access the database. Examples are as follows:

$ psql -d cbdb-database -h coordinator_host -p 5432 -U `gpadmin`

$ psql cbdb-database

$ psql

If you have not yet created a user-defined database, you can connect to the postgres database to access the system. For example:

$ psql postgres

After connecting to the database, psql displays a prompt with the name of the database to which psql is currently connected, followed by the string => (or =# if you are a database superuser). For example:

cbdb-database=>

At the prompt, you can type SQL commands. An SQL command must end with a semicolon (;`) to be sent to the server and run. For example:

=> SELECT * FROM mytable;

Connect using application interfaces

You might want to develop your own client application to interact with SynxDB. PostgreSQL provides a series of database drivers for the most common database application programming interfaces (APIs), which can also be used with SynxDB. These drivers need to be downloaded separately.

Each driver (except for libpq, which is supplied with PostgreSQL) is an independent PostgreSQL development project. You need to download, install, and configure these applications to connect to SynxDB. These drivers are as follows:

API	PostgreSQL driver	Download link
ODBC	psqlODBC	psqlODBC - PostgreSQL ODBC driver
JDBC	pgjdbc	PostgreSQL JDBC driver
Perl DBI	pgperl	DBD-Pg
Python DBI	pygresql	PyGreSQL
Python DBI	psycopg2	Psycopg
libpq C library	libpq	libpq - C Library

The general instructions for accessing SynxDB using an API are:

Download your programming language platform and the corresponding API from an appropriate source. For example, you can get the Java Development Kit (JDK) and JDBC API from Oracle.
Write your client application according to the API specification. When writing your application, be mindful of the SQL support in SynxDB, so that you do not include unsupported SQL syntax.

Download the appropriate driver and configure the connection to your SynxDB coordinator instance.

Common connection problems

There are many reasons why a client application might fail to connect to SynxDB. The following table describes some common causes of connection problems and how to resolve them.

Problem	Solution
No `pg_hba.conf` entry for host or user	To allow remote clients to connect to SynxDB, you need to configure the SynxDB coordinator instance to accept these connections. This is done by adding an appropriate entry to the `pg_hba.conf` configuration file, which is located in the data directory of the coordinator instance.
SynxDB is not running	If the SynxDB coordinator instance is stopped, users will not be able to connect. You can verify that the SynxDB system is running by running the `gpstate` utility on the SynxDB coordinator host.
Network issues: Connection timed out	If a user connects to the SynxDB coordinator host from a remote client, network issues might prevent the connection (for example, DNS hostname resolution issues, host system shutdown, and so on.). To rule out network problems, connect from the remote client host to the SynxDB coordinator host. For example: `ping hostname`. Queries and connections will fail if the system cannot resolve the host names and IP addresses of the hosts participating in SynxDB. Some operations connect to the SynxDB coordinator using `localhost`, while others use the actual host name, so you must be able to resolve both. If you encounter this error, first ensure that you can connect from the coordinator host to every host in the SynxDB list over the network. In the `/etc/hosts` file on the coordinator and all segments, ensure that you have the correct host names and IP addresses for all hosts involved in the SynxDB list. `127.0.0.1` must resolve to `localhost`.