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# frozen-string-literal: true
module Sequel
module EmulateOffsetWithReverseAndCount
# Make empty? work with an offset with an order.
# By default it would break since the order would be based on
# a column that empty does not select.
def empty?
if o = @opts[:offset]
unlimited.count <= o
else
super
end
end
# Emulate OFFSET support using reverse order in a subselect, requiring
# a count of the number of rows.
#
# If offset is used, an order must be provided, since it needs to be
# reversed in the subselect. Note that the order needs to be unambiguous
# to work correctly, and you must select all columns that you are ordering on.
def select_sql
return super if @opts[:sql]
return super unless o = @opts[:offset]
order = @opts[:order] || default_offset_order
if order.nil? || order.empty?
raise(Error, "#{db.database_type} requires an order be provided if using an offset")
end
ds = unlimited
row_count = @opts[:offset_total_count] || ds.clone(:append_sql=>String.new, :placeholder_literal_null=>true).count
dsa1 = dataset_alias(1)
if o.is_a?(Symbol) && @opts[:bind_vars] && /\A\$(.*)\z/ =~ o
# Handle use of bound variable offsets. Unfortunately, prepared statement
# bound variable offsets cannot be handled, since the bound variable value
# isn't available until later.
o = prepared_arg($1.to_sym)
end
reverse_offset = row_count - o
ds = if reverse_offset > 0
ds.limit(reverse_offset).
reverse(*order).
from_self(:alias=>dsa1).
limit(@opts[:limit]).
order(*order)
else
# Sequel doesn't allow a nonpositive limit. If the offset
# is greater than the number of rows, the empty result set
# shuld be returned, so use a condition that is always false.
ds.where(1=>0)
end
sql = @opts[:append_sql] || String.new
subselect_sql_append(sql, ds)
sql
end
# This does not support offsets in correlated subqueries, as it requires a query to get
# a count that will be invalid if a correlated subquery is used.
def supports_offsets_in_correlated_subqueries?
false
end
private
# The default order to use for datasets with offsets, if no order is defined.
# By default, orders by all of the columns in the dataset.
def default_offset_order
clone(:append_sql=>String.new, :offset=>nil).columns
end
end
end
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