epgsql

Erlang PostgreSQL Database Client

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

• 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, epgsqla and epgsqli: 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 devel branch 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

A timeout supplied at connect time works as a socket connect timeout, not a query timeout. It passes all tests from original driver except for the 3 failing timeout tests. SSL performance can degrade if the driver process has a large inbox (thousands of messages). Usage of unnamed prepared statement and portals leads to unpredicted results in case of concurrent access to same connection.

• Connect

   {ok, C} = epgsql: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

Example:

  {ok, C} = epgsql:connect("localhost", "username", [{database, "test_db"}]).
  ok = epgsql: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 epgsqli too):

    {ok, C} = epgsqla:start_link(),
    Ref = epgsqla: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}        = epgsql:squery(C, "select ...").
  {ok, Count}                = epgsql:squery(C, "update ...").
  {ok, Count, Columns, Rows} = epgsql:squery(C, "insert ... returning ...").
  
  {error, Error}             = epgsql:squery(C, "invalid SQL").
  

  • Columns - list of column records, see epgsql.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.

Several queries separated by semicolon can be executed by squery.

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

epgsqla:squery returns result as a single message:

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

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

epgsqli:squery 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 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{}}.

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 epgsql_binary.erl and the Data Representation section below.

Asynchronous api equery requires you to parse statement beforehand

    Ref = epgsqla:equery(C, Statement, [Parameters]),
    receive
      {C, Ref, Res} -> Res
    end.

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

epgsqli:equery(C, Statement, [Parameters]) sends same set of messages as squery including the 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 epgsql_types.erl.

epgsqla:parse sends {C, Ref, {ok, Statement} | {error, Reason}}. epgsqli: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 = epgsql:bind(C, Statement, [PortalName], ParameterValues).

• PortalName - optional name for the result portal.

both epgsqla:bind and epgsqli:bind 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 bind/4.
• MaxRows - maximum number of rows to return (0 for all rows).

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

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

epgsqli:execute 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 and epgsqli 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 sends {C, Ref, Results} epgsqli:execute_batch 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

  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]
  point       = {10.2, 100.12}
  
  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 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, Pid} option to epgsql:connect will result in these async messages being sent to the specified process, otherwise they will be dropped.

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 / forum

https://groups.google.com/forum/#!forum/epgsql

Test Setup

In order to run the epgsql tests, you will need to make some modifications to your local Postgres setup:

1. Add the lines at the top of test_data/test_schema.sql to your /etc/postgresql/pg_hba.conf file. Change $USER to your username.

2. Run the test_data/test_schema.sql script like so: psql template1 < test_data/test_schema.sql, as the user you intend to run the tests as.

3. make test . Currently, 6 of the tests fail.