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Sequel::Postgres::StatementCache::StatementCache

The backbone of the block, a modified LRU (least recently used) cache mapping SQL query strings to Statement objects.

Public Class Methods

new(opts={}, &block) click to toggle source

Set the options for the statement cache. These are generally set at the database level using the :statement_cache_opts Database option.

:max_size

The maximum size (high water mark) for the cache. If an entry is added when the current size of the cache is equal to the maximum size, the cache is cleaned up to reduce the number of entries to the :min_size. Defaults to 1000.

:min_size

The minimum size (low water mark) for the cache. On cleanup, the size of the cache is reduced to this number. Note that there could be fewer than this number of entries in the cache. Defaults to :max_size/2.

:prepare_after

The number of executions to wait for before preparing

the query server-side.  If set to 1, prepares all executed
queries server-side.  If set to 5, does not attempt to
prepare the query until the 5th execution.  Defaults to 2.

:sorter

A callable object that takes two arguments, the current time and the related Statement instance, and should return some Comparable (usually a numeric) such that the lowest values returned are the first to be removed when it comes time to clean the pool. The default is basically:

lambda{|t, stmt| (stmt.last_seen - t)/stmt.num_executes}

so that it doesn't remove statements that have been executed many times just because many less-frequently executed statements have been executed recently.

The block passed is called with the Statement object's name, only for statements that have been prepared, and should be used to deallocate the statements.

# File lib/sequel/extensions/pg_statement_cache.rb, line 107
def initialize(opts={}, &block)
  @cleanup_proc = block
  @prepare_after = opts.fetch(:prepare_after, 2)
  @max_size = opts.fetch(:max_size, 1000)
  @min_size = opts.fetch(:min_size, @max_size/2)
  @sorter = opts.fetch(:sorter){method(:default_sorter)}
  @ids = (1..@max_size).to_a.reverse
  @hash = {}
  #
  # We add one so that when we clean the cache, the entry
  # about to be added brings us to the min_size.
  @size_diff = @max_size - @min_size + 1
end

Public Instance Methods

clear() click to toggle source

Completely clear the statement cache, deallocating on the server side all statements that have been prepared.

# File lib/sequel/extensions/pg_statement_cache.rb, line 123
def clear
  @hash.keys.each{|k| remove(k)}
end
each(&block) click to toggle source

Yield each SQL string and Statement instance in the cache to the block.

# File lib/sequel/extensions/pg_statement_cache.rb, line 129
def each(&block)
  @hash.each(&block)
end
fetch(sql) click to toggle source

Get the related statement name from the cache. If the entry is already in the cache, just bump it's last seen time and the number of executions. Otherwise, add it to the cache. If the cache is already full, clean it up before adding it.

If the num of executions has passed the threshhold, yield the statement name to the block, which should be used to prepare the statement on the server side.

This method should return the prepared statment name if the statement has been prepared, and nil if the query has not been prepared and the statement should be executed normally.

# File lib/sequel/extensions/pg_statement_cache.rb, line 147
def fetch(sql)
  unless stmt = @hash[sql]
    # Get the next id from the id pool.
    unless id = @ids.pop
      # No id left, cache must be full, so cleanup and then
      # get the next id from the id pool.
      cleanup
      id = @ids.pop
    end
    @hash[sql] = stmt = Statement.new(id)
  end

  stmt.last_seen = Time.now
  stmt.num_executes += 1

  if stmt.num_executes >= @prepare_after
    if stmt.num_executes == @prepare_after
      begin
        yield(stmt.name)
      rescue PGError
        # An error occurred while preparing the statement,
        # execute it normally (which will probably raise
        # the error again elsewhere), but decrement the
        # number of executions so we don't think we've
        # prepared the statement when we haven't.
        stmt.num_executes -= 1
        return nil
      end
    end
    stmt.name
  end
end
size() click to toggle source

The current size of the statement cache.

# File lib/sequel/extensions/pg_statement_cache.rb, line 181
def size
  @hash.length
end

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