Erlang PostgreSQL Database Client

Asynchronous fork of https://github.com/wg/epgsql
It passes all tests from original driver except 3 timeout tests.
Difference highlights (see CHANGES for full list):
  + 3 API sets: pgsql, apgsql and ipgsql:
    pgsql maintains backwards compatibility with original driver API,
    apgsql delivers complete results as regular erlang messages,
    ipgsql delivers results as messages incrementally (row by row)
  + internal queue of client requests, so you don't need to wait for response to send next request
  + single process to hold driver state and receive socket data
  + execute several prepared statements as a batch
  + bind timestamps in erlang:now() format


* Known problems

  Timeout supplied at connect time works as socket connect timeout not query timeout.
  SSL performance degrades if driver process has large inbox (thousands of messages).


* 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 Server Notifications 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
  socket fails to connect within Timeout milliseconds.

  Asynchronous connect example (applies to ipgsql too):

  {ok, C} = apgsql:start_link(),
  Ref = apgsql:connect(C, "localhost", "username", [{database, "test_db"}]),
  receive
    {C, Ref, connected} ->
        {ok, C};
    {C, Ref, Error = {error, _}} ->
        Error;
    {'EXIT', C, _Reason} ->
        {error, closed}
  end.


* 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.

  apgsql:squery returns result as a single message:

  Ref = apgsql:squery(C, Sql),
  receive
    {C, Ref, Res} -> Res
  end.
  Res has same format as return value of pgsql:squery.

  ipgsql:squery returns result incrementally for each query inside Sql and
  for each row:

  Ref = ipgsql:squery(C, Sql),
  receive
    {C, Ref, {columns, Columns}} ->
        %% columns description
        Columns;
    {C, Ref, {data, Row}} ->
        %% single data row
        Row;
    {C, Ref, {error, _E} = Error} ->
        Error;
    {C, Ref, {complete, {_Type, Count}}} ->
        %% execution of one insert/update/delete has finished
        {ok, Count}; % affected rows count
    {C, Ref, {complete, _Type}} ->
        %% execution of one select has finished
        ok;
    {C, Ref, done} ->
        %% execution of all queries from Sql has finished
        done;
  end.


* 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.

  Ref = apgsql:equery(C, Sql, [Parameters]),
  receive
    {C, Ref, Res} -> Res
  end.
  Res has same format as return value of pgsql:equery.

  ipgsql:equery(C, Sql, [Parameters]) sends same set of messages as squery,
  including final {C, Ref, done}.


* 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.

  apgsql:parse sends {C, Ref, {ok, Statement} | {error, Reason}}.
  ipgsql:parse sends:
    {C, Ref, {types, Types}}
    {C, Ref, {columns, Columns}}
    {C, Ref, no_data} if statement will not return rows
    {C, Ref, {error, Reason}}

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

  PortalName      - optional name for the result portal.

  both apgsql:bind and ipgsql:bind send {C, Ref, ok | {error, Reason}}

  {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)

  timestamp and timestamptz parameters can take erlang:now() format {MegaSeconds, Seconds, MicroSeconds}

* 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

* Server Notifications

  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