epgsql

Erlang PostgreSQL Database Client

Asynchronous fork of wg/epgsql originally here: mabrek/epgsql and subsequently forked in order to provide a common fork for community development.

pgapp

If you want to get up to speed quickly with code that lets you run Postgres queries, you might consider trying epgsql/pgapp, which adds the following, on top of the epgsql driver:

• A 'resource pool' (currently poolboy), which lets you decide how many Postgres workers you want to utilize.
• Resilience against the database going down or other problems. The pgapp code will keep trying to reconnect to the database, but will not propagate the crash up the supervisor tree, so that, for instance, your web site will stay up even if the database is down for some reason. Erlang's "let it crash" is a good idea, but external resources going away might not be a good reason to crash your entire system.

Motivation

When you need to execute several queries, it involves a number network round-trips between the application and the database. The PostgreSQL frontend/backend protocol supports request pipelining. This means that you don't need to wait for the previous command to finish before sending the next command. This version of the driver makes full use of the protocol feature that allows faster execution.

Difference highlights

• 3 API sets:
  • epgsql maintains backwards compatibility with the original driver API
  • epgsqla delivers complete results as regular erlang messages
  • epgsqli delivers results as messages incrementally (row by row)
• internal queue of client requests, so you don't need to wait for the response to send the next request
• single process to hold driver state and receive socket data
• execution of several parsed statements as a batch
• binding timestamps in erlang:now() format

see CHANGES for full list.

Differences between current epgsql and mabrek's original async fork:

• Unnamed statements are used unless specified otherwise. This may cause problems for people attempting to use the same connection concurrently, which will no longer work.

Known problems

• SSL performance can degrade if the driver process has a large inbox (thousands of messages).

Usage

Connect

-type host() :: inet:ip_address() | inet:hostname().

-type connect_option() ::
    {database, DBName     :: string()}             |
    {port,     PortNum    :: inet:port_number()}   |
    {ssl,      IsEnabled  :: boolean() | required} |
    {ssl_opts, SslOptions :: [ssl:ssl_option()]}   | % @see OTP ssl app, ssl_api.hrl
    {timeout,  TimeoutMs  :: timeout()}            | % default: 5000 ms
    {async,    Receiver   :: pid() | atom()}       | % process to receive LISTEN/NOTIFY msgs
    {replication, Replication :: string()}. % Pass "database" to connect in replication mode
    
-spec connect(host(), string(), string(), [connect_option()] | map())
        -> {ok, Connection :: connection()} | {error, Reason :: connect_error()}.    
%% @doc connects to Postgres
%% where
%% `Host'     - host to connect to
%% `Username' - username to connect as, defaults to `$USER'
%% `Password' - optional password to authenticate with
%% `Opts'     - proplist of extra options
%% returns `{ok, Connection}' otherwise `{error, Reason}'
connect(Host, Username, Password, Opts) -> ...

example:

{ok, C} = epgsql:connect("localhost", "username", "psss", [
    {database, "test_db"},
    {timeout, 4000}
]),
...
ok = epgsql:close(C).

The {timeout, TimeoutMs} parameter will trigger an {error, timeout} result when the socket fails to connect within TimeoutMs milliseconds.

Options may be passed as map with the same key names, if your VM version supports maps.

Asynchronous connect example (applies to epgsqli too):

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

Simple Query

-type query() :: string() | iodata().
-type squery_row() :: {binary()}.

-record(column, {
    name :: binary(),
    type :: epgsql_type(),
    size :: -1 | pos_integer(),
    modifier :: -1 | pos_integer(),
    format :: integer()
}).

-type ok_reply(RowType) ::
    {ok, ColumnsDescription :: [#column{}], RowsValues :: [RowType]} |                            % select
    {ok, Count :: non_neg_integer()} |                                                            % update/insert/delete
    {ok, Count :: non_neg_integer(), ColumnsDescription :: [#column{}], RowsValues :: [RowType]}. % update/insert/delete + returning
-type error_reply() :: {error, query_error()}.
-type reply(RowType) :: ok_reply() | error_reply().

-spec squery(connection(), query()) -> reply(squery_row()) | [reply(squery_row())].
%% @doc runs simple `SqlQuery' via given `Connection'
squery(Connection, SqlQuery) -> ...

examples:

InsertRes = epgsql:squery(C, "insert into account (name) values  ('alice'), ('bob')"),
io:format("~p~n", [InsertRes]),

{ok,2}

SelectRes = epgsql:squery(C, "select * from account"),
io:format("~p~n", [SelectRes]).

{ok,
    [{column,<<"id">>,int4,4,-1,0},{column,<<"name">>,text,-1,-1,0}],
    [{<<"1">>,<<"alice">>},{<<"2">>,<<"bob">>}]
}

InsertReturningRes = epgsql:squery(C, 
    "insert into account(name)"
    "    values ('joe'), (null)"
    "    returning *"),
io:format("~p~n", [InsertReturningRes]).

{ok,2,
    [{column,<<"id">>,int4,4,-1,0}, {column,<<"name">>,text,-1,-1,0}],
    [{<<"3">>,<<"joe">>},{<<"4">>,null}]
}

{error, Reason} = epgsql:squery(C, "insert into account values (1, 'bad_pkey')"),
io:format("~p~n", [Reason]).

{error,
    error,
    <<"23505">>,
    <<"duplicate key value violates unique constraint \"account_pkey\"">>,
    [{detail,<<"Key (id)=(1) already exists.">>}]
}

The simple query protocol returns all columns as binary strings and does not support parameters binding.

Several queries separated by semicolon can be executed by squery.

  [{ok, _, [{<<"1">>}]}, {ok, _, [{<<"2">>}]}] = epgsql:squery(C, "select 1; select 2").

epgsqla:squery/2 returns result as a single message:

  Ref = epgsqla:squery(C, Sql),
  receive
    {C, Ref, Result} -> Result
  end.

Result has the same format as return value of epgsql:squery/2.

epgsqli:squery/2 returns results incrementally for each query inside Sql and for each row:

Ref = epgsqli: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 been finished
      done;
end.

Extended Query

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

SelectRes = epgsql:equery(C, "select id from account where name = $1", ["alice"]),
io:format("~p~n", [SelectRes]).

{ok,
    [{column,<<"id">>,int4,4,-1,1}],
    [{1}]
}

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

Asynchronous API epgsqla:equery/3 requires you to parse statement beforehand

#statement{types = Types} = Statement,
TypedParameters = lists:zip(Types, Parameters),
Ref = epgsqla:equery(C, Statement, [TypedParameters]),
receive
  {C, Ref, Res} -> Res
end.

• Statement - parsed statement (see parse below)
• Res has same format as return value of epgsql:equery/3.

epgsqli:equery(C, Statement, [TypedParameters]) sends same set of messages as squery including final {C, Ref, done}.

Prepared Query

{ok, Columns, Rows}        = epgsql:prepared_query(C, StatementName, [Parameters]).
{ok, Count}                = epgsql:prepared_query(C, StatementName, [Parameters]).
{ok, Count, Columns, Rows} = epgsql:prepared_query(C, StatementName, [Parameters]).
{error, Error}             = epgsql:prepared_equery(C, "non_existent_query", [Parameters]).

Parameters - optional list of values to be bound to $1, $2, $3, etc. StatementName - name of query given with erlang epgsql:parse(C, StatementName, "select ...", []).

With prepared query one can parse a query giving it a name with epgsql:parse on start and reuse the name for all further queries with different parameters.

epgsql:parse(C, "inc", "select $1+1", []).
epgsql:prepared_query(C, "inc", [4]).
epgsql:prepared_query(C, "inc", [1]).

Asynchronous API epgsqla:prepared_query/3 requires you to parse statement beforehand

#statement{types = Types} = Statement,
TypedParameters = lists:zip(Types, Parameters),
Ref = epgsqla:prepared_query(C, Statement, [TypedParameters]),
receive
  {C, Ref, Res} -> Res
end.

• Statement - parsed statement (see parse below)
• Res has same format as return value of epgsql:prepared_query/3.

epgsqli:prepared_query(C, Statement, [TypedParameters]) sends same set of messages as squery including final {C, Ref, done}.

Parse/Bind/Execute

{ok, Statement} = epgsql: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.

epgsqla:parse/2 sends {C, Ref, {ok, Statement} | {error, Reason}}. epgsqli:parse/2 sends:

• {C, Ref, {types, Types}}
• {C, Ref, {columns, Columns}}
• {C, Ref, no_data} if statement will not return rows

• {C, Ref, {error, Reason}}

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

• PortalName - optional name for the result portal.

both epgsqla:bind/3 and epgsqli:bind/3 send {C, Ref, ok | {error, Reason}}

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

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

epgsql:execute/3 returns {partial, Rows} when more rows are available.

epgsqla:execute/3 sends {C, Ref, Result} where Result has same format as return value of epgsql:execute/3.

epgsqli:execute/3 sends

• {C, Ref, {data, Row}}
• {C, Ref, {error, Reason}}
• {C, Ref, suspended} partial result was sent, more rows are available
• {C, Ref, {complete, {_Type, Count}}}

• {C, Ref, {complete, _Type}}

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

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

epgsqla/epgsqli modules' close and sync functions send {C, Ref, ok}.

Batch execution

Batch execution is bind + execute for several prepared statements. It uses unnamed portals and MaxRows = 0.

Results = epgsql:execute_batch(C, Batch).

• Batch - list of {Statement, ParameterValues}
• Results - list of {ok, Count} or {ok, Count, Rows}

example:

{ok, S1} = epgsql:parse(C, "one", "select $1", [int4]),
{ok, S2} = epgsql:parse(C, "two", "select $1 + $2", [int4, int4]),
[{ok, [{1}]}, {ok, [{3}]}] = epgsql:execute_batch(C, [{S1, [1]}, {S2, [1, 2]}]).

epgsqla:execute_batch/3 sends {C, Ref, Results} epgsqli:execute_batch/3 sends

• {C, Ref, {data, Row}}
• {C, Ref, {error, Reason}}
• {C, Ref, {complete, {_Type, Count}}}
• {C, Ref, {complete, _Type}}
• {C, Ref, done} - execution of all queries from Batch has finished

Data Representation

PG type	Representation
null	null
bool	true
char	$A
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)
point	{10.2, 100.12}
int4range	[1,5)
hstore	`{list({binary(), binary()
json/jsonb	<<"{ \"key\": [ 1, 1.0, true, \"string\" ] }">>

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

int4range is a range type for ints (bigint not supported yet) that obeys inclusive/exclusive semantics, bracket and parentheses respectively. Additionally, infinities are represented by the atoms minus_infinity and plus_infinity

Errors

Errors originating from the PostgreSQL backend are returned as {error, #error{}}, see epgsql.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 epgsql: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, PidOrName} option to epgsql:connect/3 will result in these async messages being sent to the specified pid or registered process, otherwise they will be dropped.

Another way to set notification receiver is to use set_notice_receiver/2 function. It returns previous async value. Use undefined to disable notifications.

% receiver is pid()
{ok, Previous} = epgsql:set_notice_receiver(C, self()).

% receiver is registered process
register(notify_receiver, self()).
{ok, Previous1} = epgsqla:set_notice_receiver(C, notify_receiver).

% disable notifications
{ok, Previous2} = epgsqli:set_notice_receiver(C, undefined).

Message formats:

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

• Connection - connection the notification occurred on
• Channel - channel the notification occurred on
• Pid - database session pid that sent notification

• Payload - optional payload, only available from PostgreSQL >= 9.0

  {epgsql, Connection, {notice, Error}}
  

• Connection - connection the notice occurred on

• Error - an #error{} record, see epgsql.hrl

Mailing list

Google groups

Contributing

epgsql is a community driven effort - we welcome contributions! Here's how to create a patch that's easy to integrate:

• Create a new branch for the proposed fix.
• Make sure it includes a test and documentation, if appropriate.
• Open a pull request against the devel branch of epgsql.
• Passing build in travis

Test Setup

In order to run the epgsql tests, you will need to set up a local Postgres database that runs within its own, self-contained directory, in order to avoid modifying the system installation of Postgres.

NOTE: you will need the postgis and hstore extensions to run these tests! On Ubuntu, you can install them with a command like this:

apt-get install postgresql-9.3-postgis-2.1 postgresql-contrib

1. ./setup_test_db.sh # This sets up an installation of Postgres in datadir/

2. ./start_test_db.sh # Starts a Postgres instance on its own port (10432).

3. make create_testdbs # Creates the test database environment.

4. make test # Runs the tests