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.

postgres only will denote support specific to the postgres adapter (i.e. not available when connecting to PostgreSQL via the jdbc adapter). postgres/pg only 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 driver).

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 Database#extension. 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 and support for literalizing the objects correctly for use in an SQL string, or using them as bound variable values (postgres/pg and jdbc/postgres only).

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

These extensions aren’t Database specific, they are global extensions, so you should load them via Sequel.extension, after loading support for the specific types into the Database instance:

DB.extension :pg_array
Sequel.extension :pg_array_ops

With regard to common database types, please note that the generic String type is text on PostgreSQL and not varchar(255) as it is on some other databases. text is PostgreSQL’s recommended type for storage of text data, and is more similar to Ruby’s String type as it allows for unlimited length. If you want to set a maximum size for a text column, you must specify a :size option. This will use a varchar($size) type and impose a maximum size for the column.

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 not_valid: true 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 concurrently: true 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 :using 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 Partitioned Tables¶ ↑

PostgreSQL allows marking tables as partitioned tables, and adding partitions to such tables. Sequel offers support for this. You can create a partitioned table using the :partition_by option and :partition_type options (the default partition type is range partitioning):

DB.create_table(:table1, partition_by: :column, partition_type: :range) do
  Integer :id
  Date :column
end

DB.create_table(:table2, partition_by: :column, partition_type: :list) do
  Integer :id
  String :column
end

DB.create_table(:table3, partition_by: :column, partition_type: :hash) do
  Integer :id
  Integer :column
end

To add partitions of other tables, you use the :partition_of option. This option will use a custom DSL specific to partitioning other tables. For range partitioning, you can use the from and to methods to specify the inclusive beginning and exclusive ending of the range of the partition. You can call the minvalue and maxvalue methods to get the minimum and maximum values for the column(s) in the range, useful as arguments to from and to:

DB.create_table(:table1a, partition_of: :table1) do
  from minvalue
  to 0
end
DB.create_table(:table1b, partition_of: :table1) do
  from 0
  to 100
end
DB.create_table(:table1c, partition_of: :table1) do
  from 100
  to maxvalue
end

For list partitioning, you use the values_in method. You can also use the default method to mark a partition as the default partition:

DB.create_table(:table2a, partition_of: :table2) do
  values_in 1, 2, 3
end
DB.create_table(:table2b, partition_of: :table2) do
  values_in 4, 5, 6
end
DB.create_table(:table2c, partition_of: :table2) do
  default
end

For hash partitioning, you use the modulus and remainder methods:

DB.create_table(:table3a, partition_of: :table3) do
  modulus 3
  remainder 0
end
DB.create_table(:table3b, partition_of: :table3) do
  modulus 3
  remainder 1
end
DB.create_table(:table3c, partition_of: :table3) do
  modulus 3
  remainder 2
end

There is currently no support for using custom column or table constraints in partitions of other tables. Support may be added in the future.

Creating Unlogged Tables¶ ↑

PostgreSQL allows users to create unlogged tables, which are faster but not crash safe. Sequel allows you to create an unlogged table by specifying the unlogged: true option to create_table:

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

Creating Identity Columns¶ ↑

You can use the :identity option when creating columns to mark them as identity columns. Identity columns are tied to a sequence for the default value. You can still override the default value for the column when inserting:

DB.create_table(:table){Integer :id, identity: true}
# CREATE TABLE "table" ("id" integer GENERATED BY DEFAULT AS IDENTITY)

If you want to disallow using a user provided value when inserting, you can mark the identity column using identity: :always:

DB.create_table(:table){Integer :id, identity: :always}
# CREATE TABLE "table" ("id" integer GENERATED ALWAYS AS IDENTITY)

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: :update, each_row: true, when: {Sequel[:new][:updated_at] => Sequel[:old][:updated_at]})
# CREATE TRIGGER trg_updated_at BEFORE UPDATE ON "table" FOR EACH ROW WHEN ("new"."updated_at" = "old"."updated_at") EXECUTE PROCEDURE set_updated_at()
DB.drop_trigger(:table, :trg_updated_at)
# DROP TRIGGER trg_updated_at ON "table"

However, you may want to consider just use Database#run with the necessary SQL code, at least for functions and triggers.

Parsing Check Constraints¶ ↑

Sequel has support for parsing CHECK constraints on PostgreSQL using Sequel::Database#check_constraints:

DB.create_table(:foo) do
  Integer :i
  Integer :j
  constraint(:ic, Sequel[:i] > 2)
  constraint(:jc, Sequel[:j] > 2)
  constraint(:ijc, Sequel[:i] - Sequel[:j] > 2)
end
DB.check_constraints(:foo)
# => {
#  :ic=>{:definition=>"CHECK ((i > 2))", :columns=>[:i]},
#  :jc=>{:definition=>"CHECK ((j > 2))", :columns=>[:j]},
#  :ijc=>{:definition=>"CHECK (((i - j) > 2))", :columns=>[:i, :j]}
# }

Parsing Foreign Key Constraints Referencing A Given Table¶ ↑

Sequel has support for parsing FOREIGN KEY constraints that reference a given table, using the :reverse option to foreign_key_list:

DB.create_table!(:a) do
  primary_key :id
  Integer :i
  Integer :j
  foreign_key :a_id, :a, foreign_key_constraint_name: :a_a
  unique [:i, :j]
end
DB.create_table!(:b) do
  foreign_key :a_id, :a, foreign_key_constraint_name: :a_a
  Integer :c
  Integer :d
  foreign_key [:c, :d], :a, key: [:j, :i], name: :a_c_d
end
DB.foreign_key_list(:a, reverse: true)
# => [
#  {:name=>:a_a, :columns=>[:a_id], :key=>[:id], :on_update=>:no_action, :on_delete=>:no_action, :deferrable=>false, :table=>:a, :schema=>:public},
#  {:name=>:a_a, :columns=>[:a_id], :key=>[:id], :on_update=>:no_action, :on_delete=>:no_action, :deferrable=>false, :table=>:b, :schema=>:public},
#  {:name=>:a_c_d, :columns=>[:c, :d], :key=>[:j, :i], :on_update=>:no_action, :on_delete=>:no_action, :deferrable=>false, :table=>:b, :schema=>:public}
# ]

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 Dataset#returning:

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 returning results.

VALUES Support¶ ↑

Sequel offers support for the VALUES statement using Database#values:

DB.values([[1,2],[2,3],[3,4]])
# VALUES (1, 2), (2, 3), (3, 4)

DB.values([[1,2],[2,3],[3,4]]).order(2, 1)
# VALUES (1, 2), (2, 3), (3, 4) ORDER BY 2, 1

DB.values([[1,2],[2,3],[3,4]]).order(2, 1).limit(1,2)
# VALUES (1, 2), (2, 3), (3, 4) ORDER BY 2, 1 LIMIT 1 OFFSET 2

INSERT ON CONFLICT Support¶ ↑

Starting with PostgreSQL 9.5, you can do an upsert or ignore unique or exclusion constraint violations when inserting using Dataset#insert_conflict:

DB[:table].insert_conflict.insert(a: 1, b: 2)
# INSERT INTO TABLE (a, b) VALUES (1, 2)
# ON CONFLICT DO NOTHING

For compatibility with Sequel’s MySQL support, you can also use insert_ignore:

DB[:table].insert_ignore.insert(a: 1, b: 2)
# INSERT INTO TABLE (a, b) VALUES (1, 2)
# ON CONFLICT DO NOTHING

You can pass a specific constraint name using :constraint, to only ignore a specific constraint violation:

DB[:table].insert_conflict(constraint: :table_a_uidx).insert(a: 1, b: 2)
# INSERT INTO TABLE (a, b) VALUES (1, 2)
# ON CONFLICT ON CONSTRAINT table_a_uidx DO NOTHING

If the unique or exclusion constraint covers the whole table (e.g. it isn’t a partial unique index), then you can just specify the column using the :target option:

DB[:table].insert_conflict(target: :a).insert(a: 1, b: 2)
# INSERT INTO TABLE (a, b) VALUES (1, 2)
# ON CONFLICT (a) DO NOTHING

If you want to update the existing row instead of ignoring the constraint violation, you can pass an :update option with a hash of values to update. You must pass either the :target or :constraint options when passing the :update option:

DB[:table].insert_conflict(target: :a, update: {b: Sequel[:excluded][:b]}).insert(a: 1, b: 2)
# INSERT INTO TABLE (a, b) VALUES (1, 2)
# ON CONFLICT (a) DO UPDATE SET b = excluded.b

If you want to update existing rows but using the current value of the column, you can build the desired calculation using Sequel[]

DB[:table]
  .insert_conflict(
    target: :a,
    update: {b: Sequel[:excluded][:b] + Sequel[:table][:a]}
  )
  .import([:a, :b], [ [1, 2] ])
# INSERT INTO TABLE (a, b) VALUES (1, 2)
# ON CONFLICT (a) DO UPDATE SET b = (excluded.b + table.a)

Additionally, if you only want to do the update in certain cases, you can specify an :update_where option, which will be used as a filter. If the row doesn’t match the conditions, the constraint violation will be ignored, but the row will not be updated:

DB[:table].insert_conflict(constraint::table_a_uidx,
  update: {b: Sequel[:excluded][:b]},
  update_where: {Sequel[:table][:status_id]=>1}).insert(a: 1, b: 2)
# INSERT INTO TABLE (a, b) VALUES (1, 2)
# ON CONFLICT ON CONSTRAINT table_a_uidx
# DO UPDATE SET b = excluded.b WHERE (table.status_id = 1)

INSERT OVERRIDING SYSTEM|USER VALUE Support¶ ↑

PostgreSQL 10+ supports identity columns, which are designed to replace the serial columns previously used for autoincrementing primary keys. You can use Dataset#overriding_system_value and Dataset#overriding_user_value to use this new syntax:

DB.create_table(:table){primary_key :id}
# Ignore the given value for id, using the identity's sequence value.
DB[:table].overriding_user_value.insert(id: 1)

DB.create_table(:table){primary_key :id, identity: :always}
# Force the use of the given value for id, because otherwise the insert will
# raise an error, since GENERATED ALWAYS was used when creating the column.
DB[:table].overriding_system_value.insert(id: 1)

Distinct On Specific Columns¶ ↑

Sequel allows passing columns to Dataset#distinct, 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"

JOIN USING table alias¶ ↑

Sequel allows passing an SQL::AliasedExpression to join table methods to use a USING join with a table alias for the USING columns:

DB[:t1].join(:t2, Sequel.as([:c1, :c2], :alias))
# SELECT * FROM "t1" INNER JOIN "t2" USING ("c1", "c2") AS "alias"

Calling PostgreSQL 11+ Procedures `postgres only`¶ ↑

PostgreSQL 11+ added support for procedures, which are different from the user defined functions that PostgreSQL has historically supported. These procedures are called via a special CALL syntax, and Sequel supports them via Database#call_procedure:

DB.call_procedure(:foo, 1, "bar")
# CALL foo(1, 'bar')

Database#call_procedure will return a hash of return values if the procedure returns a result, or nil if the procedure does not return a result.

Using a Cursor to Process Large Datasets `postgres only`¶ ↑

The postgres adapter offers a Dataset#use_cursor 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 Dataset#paged_each.

Using cursors, it is possible to update individual rows of a large dataset easily using the rows_per_fetch: 1 option in conjunction with Dataset#where_current_of. 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 `postgres/pg and jdbc/postgres only` ¶ ↑

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 Database#copy_table, 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 Database#copy_into:

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 Database#do (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 Database#notify to send notification to channels:

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

postgres/pg only You can listen on channels via Database#listen. 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;

Extended Error Info (`postgres/pg only`)¶ ↑

If you run a query that raises a Sequel::DatabaseError, you can pass the exception object to Database#error_info, and that will return a hash with metadata regarding the error, such as the related table and column or constraint.

DB.create_table(:test1){primary_key :id}
DB.create_table(:test2){primary_key :id; foreign_key :test1_id, :test1}
DB[:test2].insert(test1_id: 1) rescue DB.error_info($!)
# => {
#  :schema=>"public",
#  :table=>"test2",
#  :column=>nil,
#  :constraint=>"test2_test1_id_fkey",
#  :type=>nil,
#  :severity=>"ERROR",
#  :sql_state=>"23503",
#  :message_primary=>"insert or update on table \"test2\" violates foreign key constraint \"test2_test1_id_fkey\"",
#  :message_detail=>"Key (test1_id)=(1) is not present in table \"test1\"."
#  :message_hint=>nil,
#  :statement_position=>nil,
#  :internal_position=>nil,
#  :internal_query=>nil,
#  :source_file=>"ri_triggers.c",
#  :source_line=>"3321",
#  :source_function=>"ri_ReportViolation"
# }

sequel_pg (`postgres/pg only`)¶ ↑

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 ~2x 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, as_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, Dataset#paged_each will use streaming to implement paging.

postgresql.rdoc

PostgreSQL-specific Support in Sequel¶ ↑