class Sequel::Postgres::Dataset
Constants
- BindArgumentMethods
- PREPARED_ARG_PLACEHOLDER
:nocov:
- PreparedStatementMethods
Public Instance Methods
# File lib/sequel/adapters/postgres.rb 738 def bound_variable_modules 739 [BindArgumentMethods] 740 end
# File lib/sequel/adapters/postgres.rb 649 def fetch_rows(sql) 650 return cursor_fetch_rows(sql){|h| yield h} if @opts[:cursor] 651 execute(sql){|res| yield_hash_rows(res, fetch_rows_set_cols(res)){|h| yield h}} 652 end
Use a cursor for paging.
# File lib/sequel/adapters/postgres.rb 655 def paged_each(opts=OPTS, &block) 656 unless defined?(yield) 657 return enum_for(:paged_each, opts) 658 end 659 use_cursor(opts).each(&block) 660 end
PostgreSQL uses $N for placeholders instead of ?, so use a $ as the placeholder.
# File lib/sequel/adapters/postgres.rb 748 def prepared_arg_placeholder 749 PREPARED_ARG_PLACEHOLDER 750 end
# File lib/sequel/adapters/postgres.rb 742 def prepared_statement_modules 743 [PreparedStatementMethods] 744 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.
- :skip_transaction
-
Same as :hold, but :hold takes priority.
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.
# File lib/sequel/adapters/postgres.rb 685 def use_cursor(opts=OPTS) 686 clone(:cursor=>{:rows_per_fetch=>1000}.merge!(opts)) 687 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
# File lib/sequel/adapters/postgres.rb 697 def where_current_of(cursor_name='sequel_cursor') 698 clone(:where=>Sequel.lit(['CURRENT OF '], Sequel.identifier(cursor_name))) 699 end