= PostgreSQL-specific Support in Sequel

Sequel's core database and dataset functions are designed to support the features
shared by most common SQL database implementations.  However, Sequel's database
adapters extend the core support to include support for database-specific features.

By far the most extensive database-specific support in Sequel is for PostgreSQL. This
support is roughly broken into the following areas:

* Database Types
* DDL Support
* DML Support
* sequel_pg

Note that while this guide is extensive, it is not exhaustive.  There are additional
rarely used PostgreSQL features that Sequel supports which are not mentioned here.

== Adapter/Driver Specific Support

Some of this this support depends on the specific adapter or underlying driver in use.  
<tt>postgres only</tt> will denote support specific to the postgres adapter (i.e.
not available when connecting to PostgreSQL via the jdbc, do, or swift adapters).
<tt>postgres/pg only</tt> will denote support specific to the postgres adapter when
pg is used as the underlying driver (i.e. not available when using the postgres-pr or
postgres drivers).

== PostgreSQL-specific Database Type Support

Sequel's default support on PostgreSQL only includes common database types.  However,
Sequel ships with support for many PostgreSQL-specific types via extensions.  In general,
you load these extensions via <tt>Database#extension</tt>.  For example, to load support
for arrays, you would do:

  DB.extension :pg_array

The following PostgreSQL-specific type extensions are available:

pg_array :: arrays (single and multidimensional, for any scalar type), as a ruby Array-like object
pg_hstore :: hstore, as a ruby Hash-like object
pg_inet :: inet/cidr, as ruby IPAddr objects
pg_interval :: interval, as ActiveSupport::Duration objects
pg_json :: json, as either ruby Array-like or Hash-like objects
pg_range :: ranges (for any scalar type), as a ruby Range-like object
pg_row :: row-valued/composite types, as a ruby Hash-like or Sequel::Model object

In general, these extensions just add support for Database objects to return retrieved
column values as the appropriate type (<tt>postgres and jdbc/postgres only</tt>), and support for literalizing
the objects correctly for use in an SQL string, or using them as bound variable values (<tt>postgres/pg and jdbc/postgres only</tt>).

There are also type-specific extensions that make it easy to use database functions
and operators related to the type.  These extensions are:

pg_array_ops :: array-related functions and operators
pg_hstore_ops :: hstore-related functions and operators
pg_json_ops :: json-related functions and operators
pg_range_ops :: range-related functions and operators
pg_row_ops :: row-valued/composite type syntax support

== PostgreSQL-specific DDL Support

=== Exclusion Constraints

In +create_table+ blocks, you can use the +exclude+ method to set up exclusion constraints:

  DB.create_table(:table) do
    daterange :during
    exclude([[:during, '&&']], :name=>:table_during_excl)
  end
  # CREATE TABLE "table" ("during" daterange,
  #   CONSTRAINT "table_during_excl" EXCLUDE USING gist ("during" WITH &&))

You can also add exclusion constraints in +alter_table+ blocks using add_exclusion_constraint:

  DB.alter_table(:table) do
    add_exclusion_constraint([[:during, '&&']], :name=>:table_during_excl)
  end
  # ALTER TABLE "table" ADD CONSTRAINT "table_during_excl" EXCLUDE USING gist ("during" WITH &&)

=== Adding Foreign Key and Check Constraints Without Initial Validation

You can add a <tt>:not_valid=>true</tt> option when adding constraints to existing tables so
that it doesn't check if all current rows are valid:

  DB.alter_table(:table) do
    # Assumes t_id column already exists
    add_foreign_key([:t_id], :table, :not_valid=>true, :name=>:table_fk)

    constraint({:name=>:col_123, :not_valid=>true}, :col=>[1,2,3])
  end
  # ALTER TABLE "table" ADD CONSTRAINT "table_fk" FOREIGN KEY ("t_id") REFERENCES "table" NOT VALID
  # ALTER TABLE "table" ADD CONSTRAINT "col_123" CHECK (col IN (1, 2, 3)) NOT VALID

Such constraints will be enforced for newly inserted and updated rows, but not for existing rows. After
all existing rows have been fixed, you can validate the constraint:

  DB.alter_table(:table) do
    validate_constraint(:table_fk)
    validate_constraint(:col_123)
  end
  # ALTER TABLE "table" VALIDATE CONSTRAINT "table_fk"
  # ALTER TABLE "table" VALIDATE CONSTRAINT "col_123"

=== Creating Indexes Concurrently

You can create indexes concurrently using the <tt>:concurrently=>true</tt> option:

  DB.add_index(:table, :t_id, :concurrently=>true)
  # CREATE INDEX CONCURRENTLY "table_t_id_index" ON "table" ("t_id")

Similarly, you can drop indexes concurrently as well:

  DB.drop_index(:table, :t_id, :concurrently=>true)
  # DROP INDEX CONCURRENTLY "table_t_id_index"

=== Specific Conversions When Altering Column Types

When altering a column type, PostgreSQL allows the user to specify how to do the
conversion via a USING clause, and Sequel supports this using the <tt>:using</tt> option:

  DB.alter_table(:table) do
    # Assume unix_time column is stored as an integer, and you want to change it to timestamp
    set_column_type :unix_time, Time, :using=>(Sequel.cast('epoch', Time) + Sequel.cast('1 second', :interval) * :unix_time)
  end
  # ALTER TABLE "table" ALTER COLUMN "unix_time" TYPE timestamp
  #   USING (CAST('epoch' AS timestamp) + (CAST('1 second' AS interval) * "unix_time"))

=== Creating Unlogged Tables

PostgreSQL allows users to create unlogged tables, which are faster but not crash safe.  Sequel
allows you do create an unlogged table by specifying the <tt>:unlogged=>true</tt> option to +create_table+:

  DB.create_table(:table, :unlogged=>true){Integer :i}
  # CREATE UNLOGGED TABLE "table" ("i" integer)

=== Creating/Dropping Schemas, Languages, Functions, and Triggers

Sequel has built in support for creating and dropping PostgreSQL schemas, procedural languages, functions, and triggers:

  DB.create_schema(:s)
  # CREATE SCHEMA "s"
  DB.drop_schema(:s)
  # DROP SCHEMA "s"

  DB.create_language(:plperl)
  # CREATE LANGUAGE plperl
  DB.drop_language(:plperl)
  # DROP LANGUAGE plperl

  DB.create_function(:set_updated_at, <<-SQL, :language=>:plpgsql, :returns=>:trigger)
    BEGIN
      NEW.updated_at := CURRENT_TIMESTAMP;
      RETURN NEW;
    END;
  SQL
  # CREATE FUNCTION set_updated_at() RETURNS trigger LANGUAGE plpgsql AS '
  #  BEGIN
  #    NEW.updated_at := CURRENT_TIMESTAMP;
  #    RETURN NEW;
  #  END;'
  DB.drop_function(:set_updated_at)
  # DROP FUNCTION set_updated_at()

  DB.create_trigger(:table, :trg_updated_at, :set_updated_at, :events=>[:insert, :update], :each_row=>true)
  # CREATE TRIGGER trg_updated_at BEFORE INSERT OR UPDATE ON "table" FOR EACH ROW EXECUTE PROCEDURE set_updated_at()
  DB.drop_trigger(:table, :trg_updated_at)
  # DROP TRIGGER trg_updated_at ON "table"

== PostgreSQL-specific DML Support

=== Returning Rows From Insert, Update, and Delete Statements

Sequel supports the ability to return rows from insert, update, and delete statements, via
<tt>Dataset#returning</tt>:

  DB[:table].returning.insert
  # INSERT INTO "table" DEFAULT VALUES RETURNING *

  DB[:table].returning(:id).delete
  # DELETE FROM "table" RETURNING "id"

  DB[:table].returning(:id, Sequel.*(:id, :id).as(:idsq)).update(:id=>2)
  # UPDATE "table" SET "id" = 2 RETURNING "id", ("id" * "id") AS "idsq"

When returning is used, instead of returning the number of rows affected (for updated/delete)
or the serial primary key value (for insert), it will return an array of hashes with the
returned results.

=== Distinct On Specific Columns

Sequel allows passing columns to <tt>Dataset#distinct</tt>, which will make the dataset return
rows that are distinct on just those columns:

  DB[:table].distinct(:id).all
  # SELECT DISTINCT ON ("id") * FROM "table"

=== Using a Cursor to Process Large Datasets <tt>postgres only</tt>

The postgres adapter offers a <tt>Dataset#use_cursor</tt> method to process large result sets
without keeping all rows in memory:

  DB[:table].use_cursor.each{|row| }
  # BEGIN;
  # DECLARE sequel_cursor NO SCROLL CURSOR WITHOUT HOLD FOR SELECT * FROM "table";
  # FETCH FORWARD 1000 FROM sequel_cursor
  # FETCH FORWARD 1000 FROM sequel_cursor
  # ...
  # FETCH FORWARD 1000 FROM sequel_cursor
  # CLOSE sequel_cursor
  # COMMIT

This support is used by default when using <tt>Dataset#paged_each</tt>.

Using cursors, it is possible to update individual rows of a large dataset
easily using the <tt>:rows_per_fetch=>1</tt> option in conjunction with
<tt>Dataset#where_current_of</tt>.  This is useful if the logic needed to
update the rows exists in the application and not in the database:

  ds.use_cursor(:rows_per_fetch=>1).each do |row|
    ds.where_current_of.update(:col=>new_col_value(row))
  end

=== Truncate Modifiers

Sequel supports PostgreSQL-specific truncate options:

  DB[:table].truncate(:cascade => true, :only=>true, :restart=>true)
  # TRUNCATE TABLE ONLY "table" RESTART IDENTITY CASCADE

=== COPY Support <tt>postgres/pg and jdbc/postgres only</tt> 

PostgreSQL's COPY feature is pretty much the fastest way to get data in or out of the database.
Sequel supports getting data out of the database via <tt>Database#copy_table</tt>, either for
a specific table or for an arbitrary dataset:

  DB.copy_table(:table, :format=>:csv)
  # COPY "table" TO STDOUT (FORMAT csv)
  DB.copy_table(DB[:table], :format=>:csv)
  # COPY (SELECT * FROM "table") TO STDOUT (FORMAT csv)

It supports putting data into the database via <tt>Database#copy_into</tt>:

  DB.copy_into(:table, :format=>:csv, :columns=>[:column1, :column2], :data=>"1,2\n2,3\n")
  # COPY "table"("column1", "column2") FROM STDIN (FORMAT csv)

=== Anonymous Function Execution

You can execute anonymous functions using a database procedural language via <tt>Database#do</tt> (the
plpgsql language is the default):

  DB.do <<-SQL
    DECLARE r record;
    BEGIN
     FOR r IN SELECT table_schema, table_name FROM information_schema.tables
       WHERE table_type = 'VIEW' AND table_schema = 'public'
     LOOP
       EXECUTE 'GRANT ALL ON ' || quote_ident(r.table_schema) || '.' || quote_ident(r.table_name) || ' TO webuser';
     END LOOP;
    END;
  SQL

=== Listening On and Notifying Channels

You can use <tt>Database#notify</tt> to send notification to channels:

  DB.notify(:channel)
  # NOTIFY "channel"

<tt>postgres/pg only</tt> You can listen on channels via <tt>Database#listen</tt>.  Note that
this blocks until the listening thread is notified:

  DB.listen(:channel)
  # LISTEN "channel"
  # after notification received:
  # UNLISTEN *

Note that +listen+ by default only listens for a single notification.  If you want to loop and process
notifications:

  DB.listen(:channel, :loop=>true){|channel| p channel}

The +pg_static_cache_updater+ extension uses this support to automatically update
the caches for models using the +static_cache+ plugin.  Look at the documentation of that
plugin for details.

=== Locking Tables

Sequel makes it easy to lock tables, though it is generally better to let the database
handle locking:

  DB[:table].lock('EXCLUSIVE') do
    DB[:table].insert(:id=>DB[:table].max(:id)+1)
  end
  # BEGIN;
  # LOCK TABLE "table" IN EXCLUSIVE MODE;
  # SELECT max("id") FROM "table" LIMIT 1;
  # INSERT INTO "table" ("id") VALUES (2) RETURNING NULL;
  # COMMIT;

== sequel_pg (<tt>postgres/pg only</tt>)

When the postgres adapter is used with the pg driver, Sequel automatically checks for sequel_pg, and
loads it if it is available.  sequel_pg is a C extension that optimizes the fetching of rows, generally
resulting in a 2-6x speedup.  It is highly recommended to install sequel_pg if you are using the
postgres adapter with pg.

sequel_pg has additional optimizations when using the Dataset +map+, +to_hash+,
+to_hash_groups+, +select_hash+, +select_hash_groups+, +select_map+, and +select_order_map+ methods,
which avoids creating intermediate hashes and can add further speedups.

In addition to optimization, sequel_pg also adds streaming support if used on PostgreSQL 9.2. Streaming
support is similar to using a cursor, but it is faster and more transparent.

You can enable the streaming support:

  DB.extension(:pg_streaming)

Then you can stream individual datasets:

  DB[:table].stream.each{|row| }

Or stream all datasets by default:

  DB.stream_all_queries = true

When streaming is enabled, <tt>Dataset#paged_each</tt> will use streaming to implement
paging.