Class Sequel::Postgres::Dataset
In: lib/sequel/adapters/postgres.rb
Parent: Sequel::Dataset

Methods

Included Modules

Sequel::Postgres::DatasetMethods

Classes and Modules

Module Sequel::Postgres::Dataset::ArgumentMapper

Constants

PREPARED_ARG_PLACEHOLDER = LiteralString.new('$').freeze
BindArgumentMethods = prepared_statements_module(:bind, [ArgumentMapper], %w'execute execute_dui')
PreparedStatementMethods = prepared_statements_module(:prepare, BindArgumentMethods, %w'execute execute_dui')

Public Instance methods

[Source]

     # File lib/sequel/adapters/postgres.rb, line 727
727:         def bound_variable_modules
728:           [BindArgumentMethods]
729:         end

[Source]

     # File lib/sequel/adapters/postgres.rb, line 639
639:       def fetch_rows(sql)
640:         return cursor_fetch_rows(sql){|h| yield h} if @opts[:cursor]
641:         execute(sql){|res| yield_hash_rows(res, fetch_rows_set_cols(res)){|h| yield h}}
642:       end

Use a cursor for paging.

[Source]

     # File lib/sequel/adapters/postgres.rb, line 645
645:       def paged_each(opts=OPTS, &block)
646:         use_cursor(opts).each(&block)
647:       end

PostgreSQL uses $N for placeholders instead of ?, so use a $ as the placeholder.

[Source]

     # File lib/sequel/adapters/postgres.rb, line 737
737:         def prepared_arg_placeholder
738:           PREPARED_ARG_PLACEHOLDER
739:         end

[Source]

     # File lib/sequel/adapters/postgres.rb, line 731
731:         def prepared_statement_modules
732:           [PreparedStatementMethods]
733:         end

Uses a cursor for fetching records, instead of fetching the entire result set at once. Note this uses a transaction around the cursor usage by default and can be changed using `hold: true` as described below. Cursors can be used to process large datasets without holding all rows in memory (which is what the underlying drivers may do by default). Options:

:cursor_name :The name assigned to the cursor (default ‘sequel_cursor’). Nested cursors require different names.
:hold :Declare the cursor WITH HOLD and don‘t use transaction around the cursor usage.
:rows_per_fetch :The number of rows per fetch (default 1000). Higher numbers result in fewer queries but greater memory use.

Usage:

  DB[:huge_table].use_cursor.each{|row| p row}
  DB[:huge_table].use_cursor(rows_per_fetch: 10000).each{|row| p row}
  DB[:huge_table].use_cursor(cursor_name: 'my_cursor').each{|row| p row}

This is untested with the prepared statement/bound variable support, and unlikely to work with either.

[Source]

     # File lib/sequel/adapters/postgres.rb, line 671
671:       def use_cursor(opts=OPTS)
672:         clone(:cursor=>{:rows_per_fetch=>1000}.merge!(opts))
673:       end

Replace the WHERE clause with one that uses CURRENT OF with the given cursor name (or the default cursor name). This allows you to update a large dataset by updating individual rows while processing the dataset via a cursor:

  DB[:huge_table].use_cursor(rows_per_fetch: 1).each do |row|
    DB[:huge_table].where_current_of.update(column: ruby_method(row))
  end

[Source]

     # File lib/sequel/adapters/postgres.rb, line 683
683:       def where_current_of(cursor_name='sequel_cursor')
684:         clone(:where=>Sequel.lit(['CURRENT OF '], Sequel.identifier(cursor_name)))
685:       end

[Validate]