wg/epgsql

Erlang PostgreSQL Database Client

• Connect

  {ok, C} = pgsql:connect(Host, [Username], [Password], Opts).

  Host - host to connect to. Username - username to connect as, defaults to $USER. Password - optional password to authenticate with. Opts - property list of extra options. Supported properties:

  • {database, String}
  • {port, Integer}
  • {ssl, Atom} true | false | required
  • {ssl_opts, List} see ssl application docs in OTP
  • {timeout, Integer} milliseconds, defaults to 5000
  • {async, Pid} see Asynchronous Messages section

  {ok, C} = pgsql:connect("localhost", "username", [{database, "test_db"}]). ok = pgsql:close(C).

  The timeout parameter will trigger an {error, timeout} result when the server fails to respond within Timeout milliseconds. This timeout applies to the initial connection attempt and any subsequent queries.

• Simple Query

  {ok, Columns, Rows} = pgsql:squery(C, "select ..."). {ok, Count} = pgsql:squery(C, "update ..."). {ok, Count, Columns, Rows} = pgsql:squery(C, "insert ... returning ...").

  {error, Error} = pgsql:squery(C, "invalid SQL").

  Columns - list of column records, see pgsql.hrl for definition. Rows - list of tuples, one for each row. Count - integer count of rows inserted/updated/etc

  The simple query protocol returns all columns as text (Erlang binaries) and does not support binding parameters.

• Extended Query

  {ok, Columns, Rows} = pgsql:equery(C, "select ...", [Parameters]). {ok, Count} = pgsql:equery(C, "update ...", [Parameters]). {ok, Count, Columns, Rows} = pgsql:equery(C, "insert ... returning ...", [Parameters]).

  {error, Error} = pgsql:equery(C, "invalid SQL", [Parameters]).

  Parameters - optional list of values to be bound to $1, $2, $3, etc.

  The extended query protocol combines parse, bind, and execute using the unnamed prepared statement and portal. A "select" statement returns {ok, Columns, Rows}, "insert/update/delete" returns {ok, Count} or {ok, Count, Columns, Rows} when a "returning" clause is present. When an error occurs, all statements result in {error, #error{}}.

  PostgreSQL's binary format is used to return integers as Erlang integers, floats as floats, bytea/text/varchar columns as binaries, bools as true/false, etc. For details see pgsql_binary.erl and the Data Representation section below.

• Parse/Bind/Execute

  {ok, Statement} = pgsql:parse(C, [StatementName], Sql, [ParameterTypes]).

  StatementName - optional, reusable, name for the prepared statement. ParameterTypes - optional list of PostgreSQL types for each parameter.

  For valid type names see pgsql_types.erl.

  ok = pgsql:bind(C, Statement, [PortalName], ParameterValues).

  PortalName - optional name for the result portal.

  {ok | partial, Rows} = pgsql:execute(C, Statement, [PortalName], [MaxRows]). {ok, Count} = pgsql:execute(C, Statement, [PortalName]). {ok, Count, Rows} = pgsql:execute(C, Statement, [PortalName]).

  PortalName - optional portal name used in bind/4. MaxRows - maximum number of rows to return (0 for all rows).

  execute returns {partial, Rows} when more rows are available.

  ok = pgsql:close(C, Statement). ok = pgsql:close(C, statement | portal, Name). ok = pgsql:sync(C).

  All functions return {error, Error} when an error occurs.

• Data Representation

  null = null bool = true | false char = $A | binary intX = 1 floatX = 1.0 date = {Year, Month, Day} time = {Hour, Minute, Second.Microsecond} timetz = {time, Timezone} timestamp = {date, time} timestamptz = {date, time} interval = {time, Days, Months} text = <<"a">> varchar = <<"a">> bytea = <<1, 2>> array = [1, 2, 3]

  record = {int2, time, text, ...} (decode only)

• Errors

  Errors originating from the PostgreSQL backend are returned as {error, #error{}}, see pgsql.hrl for the record definition. epgsql functions may also return {error, What} where What is one of the following:

  {unsupported_auth_method, Method} - required auth method is unsupported timeout - request timed out closed - connection was closed sync_required - error occured and pgsql:sync must be called

• Asynchronous Messages

  PostgreSQL may deliver two types of asynchronous message: "notices" in response to notice and warning messages generated by the server, and "notifications" which are generated by the LISTEN/NOTIFY mechanism.

  Passing the {async, Pid} option to pgsql:connect will result in these async messages being sent to the specified process, otherwise they will be dropped.

  Message formats:

  {pgsql, Connection, {notification, Channel, Pid, Payload}}

  Connection  - connection the notification occured on
  
  Channel     - channel the notification occured on
  Pid         - database session pid that sent notification
  Payload     - optional payload, only available from PostgreSQL >= 9.0
  

  {pgsql, Connection, {notice, Error}}

  Connection  - connection the notice occured on
  Error       - an #error{} record, see pgsql.hrl