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:
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Database Types
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DDL Support
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DML Support
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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.
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, do, or swift adapters). 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 or postgres drivers).
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
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arrays (single and multidimensional, for any scalar type), as a ruby Array-like object
- pg_hstore
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hstore, as a ruby Hash-like object
- pg_inet
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inet/cidr, as ruby IPAddr objects
- pg_interval
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interval, as ActiveSupport::Duration objects
- pg_json
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json, as either ruby Array-like or Hash-like objects
- pg_range
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ranges (for any scalar type), as a ruby Range-like object
- pg_row
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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 (postgres only), and support for literalizing the objects correctly for use in an SQL string, or using them as bound variable values (postgres/pg 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
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array-related functions and operators
- pg_hstore_ops
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hstore-related functions and operators
- pg_range_ops
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range-related functions and operators
- pg_row_ops
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row-valued/composite type syntax support
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 &&)
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) end # ALTER TABLE "table" ADD CONSTRAINT "table_fk" FOREIGN KEY ("t_id") REFERENCES "table" 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) end # ALTER TABLE "table" VALIDATE CONSTRAINT "table_fk"
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"
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"))
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 :unlogged=>true option to create_table:
DB.create_table(:table, :unlogged=>true){Integer :i} # CREATE UNLOGGED TABLE "table" ("i" integer)
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"
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 returned results.
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"
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
Sequel supports PostgreSQL-specific truncate options:
DB[:table].truncate(:cascade => true, :only=>true, :restart=>true) # TRUNCATE TABLE ONLY "table" RESTART IDENTITY CASCADE
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)
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
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}
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;
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