NewSessionStatsList initializes a new SessionStatsList.

NewStatsUsage creates a new StatsUsage.

NewStatsUsageImpl creates a statstypes.StatsUsage.

NewTableDelta creates a new TableDelta.

UpdateTableDeltaMap updates the delta of the table.

DumpStatsDeltaRatio is the lower bound of `Modify Count / Table Count` for stats delta to be dumped.

SessionStatsItem is a list item that holds the delta mapper.

SessionStatsList is a list of SessionStatsItem, which is used to collect stats usage and table delta information from sessions.

StatsUsage maps (tableID, columnID) to the last time when the column stats are used(needed).

TableDelta is used to collect tables' change information.