crystal-pg

A native, non-blocking Postgres driver for Crystal

usage

This driver now uses the crystal-db project. Documentation on connecting,querying, etc, can be found at:

• https://crystal-lang.org/docs/database/
• https://crystal-lang.org/docs/database/connection_pool.html

shards

Add this to your shard.yml and run shards install

dependencies:
  pg:
    github: will/crystal-pg

Listen/Notify

There are two ways to listen for notifications. For docs on NOTIFY, pleaseread https://www.postgresql.org/docs/current/static/sql-notify.html.

1. Any connection can be given a callback to run on notifications. However theyare only received when other traffic is going on.
2. A special listen-only connection can be established for instant notificationprocessing with PG.connect_listen.

# see full example in examples/listen_notify.cr
PG.connect_listen("postgres:///", "a", "b") do |n| # connect and  listen on "a" and "b"
  puts "    got: #{n.payload} on #{n.channel}"     # print notifications as they come in
end

Arrays

Crystal-pg supports several popular array types. If you only need a 1dimensional array, you can cast down to the appropriate Crystal type:

PG_DB.query_one("select ARRAY[1, null, 3]", &.read(Array(Int32?))
# => [1, nil, 3]

PG_DB.query_one("select '{hello, world}'::text[]", &.read(Array(String))
# => ["hello", "world"]

Error Handling

It is possible to catch errors and notifications and pass them along to Crystal for further handling.

DB.connect("postgres:///") do |cnn|
  # Capture and print all exceptions
  cnn.on_notice { |x| puts "pgSQL #{x}" }

  # A function that raises exceptions
  cnn.exec(
    <<-SQL
      CREATE OR REPLACE FUNCTION foo(IN str TEXT)
        RETURNS VOID
        LANGUAGE 'plpgsql'
        AS $$
          BEGIN
            IF str = 'yes' THEN
                    RAISE NOTICE 'Glad we agree!';
            ELSE
              RAISE EXCEPTION 'You know nothing John Snow!';
            END IF;
          END;
        $$;
    SQL
  )

  # Notice handling example
  cnn.exec(
    <<-SQL
      SELECT foo('yes');
    SQL
  )
  # => pgSQL NOTICE: Glad we agree!

  # Exception handling example
  cnn.exec(
    <<-SQL
      SELECT foo('no');
    SQL
  )
  # => pgSQL ERROR: You know nothing John Snow!
  #    Unhandled exception: You know nothing John Snow! (PQ::PQError)
  #     from lib/pg/src/pq/connection.cr:203:7 in 'handle_error'
  #     from lib/pg/src/pq/connection.cr:186:7 in 'handle_async_frames'
  #     from lib/pg/src/pq/connection.cr:162:7 in 'read'
  #     from lib/pg/src/pq/connection.cr:386:18 in 'expect_frame'
  #     from lib/pg/src/pq/connection.cr:370:9 in 'read_next_row_start'
  #     from lib/pg/src/pg/result_set.cr:39:8 in 'move_next'
  #     from lib/pg/src/pg/statement.cr:39:13 in 'perform_exec'
  #     from lib/db/src/db/statement.cr:82:14 in 'perform_exec_and_release'
  #     from lib/db/src/db/statement.cr:68:7 in 'exec:args'
  #     from lib/db/src/db/query_methods.cr:271:7 in 'exec'
  #     from spec/cerebrum_spec.cr:84:3 in '__crystal_main'
  #     from /usr/share/crystal/src/crystal/main.cr:97:5 in 'main_user_code'
  #     from /usr/share/crystal/src/crystal/main.cr:86:7 in 'main'
  #     from /usr/share/crystal/src/crystal/main.cr:106:3 in 'main'
  #     from __libc_start_main
  #     from _start
  #     from ???

Requirements

Crystal-pg is regularly tested onthe Postgres versions the Postgres project itself supports.Since it uses protocol version 3, older versions probably also work but are not guaranteed.

Supported Datatypes

• text
• boolean
• int8, int4, int2
• float4, float8
• timestamptz, date, timestamp (but no one should use ts when tstz exists!)
• json and jsonb
• uuid
• bytea
• numeric/decimal (1)
• varchar
• regtype
• geo types: point, box, path, lseg, polygon, circle, line
• array types: int8, int4, int2, float8, float4, bool, text, numeric, timestamptz, date, timestamp
• interval (2)

1: A note on numeric: In Postgres this type has arbitrary precision. In thisdriver, it is represented as a PG::Numeric which retains all precision, butif you need to do any math on it, you will probably need to cast it to afloat first. If you need true arbitrary precision, you can optionallyrequire pg_ext/big_rational which adds #to_big_r, but requires that youhave LibGMP installed.

2: A note on interval: A Postgres interval can not be directly mapped to a builtin Crystal datatype. Therfore we provide a PG::Interval type that can be converted toTime::Span and Time::MonthSpan.

Authentication Methods

By default this driver will accept scram-sha-256 and md5, as well astrust. However cleartext is disabled by default. You can control exactlywhich auth methods the client will accept by passing in a comma separated listto the auth_methods parameter, for example

DB.open("postgres://example.com/dbname?auth_methods=cleartext,md5,scram-sha-256")

DO NOT TURN cleartext ON UNLESS YOU ABSOLUTELY NEED IT! Mearly by havingthis option enabled exposes a postgres client to downgrade man-in-the-middleattacks, even if the server is configured to not support cleartext. Even if youuse TLS, you are not safe unless you are fully verifying the server's cert, asthe attacker can terminate TLS and re-negotiate a connection with the server.

client                     attacker                     server
----------------------------------------------------------------------------
I want to connect \
                   \->  intercepts, forwards
                        I want to connect \
                                           \----->  receives connection request

                                                  / I support scram and/or md5 only
                        intercepts, sends      <-/
                     /  I only support cleartext
receives attacker <-/
claiming server
only supports cleartext
sends password because
cleartext enabled \
                   \->  receives clear password,
                        negotiates scram/md5
                        with real server      \
                                               \--> accepts scram/md5 auth

It is a mistake for any driver to support cleartext by default, and it's amistake that postgres continues to have this as an option at all.