CloneColInfos uses (*ColumnInfo).Clone to clone a slice of *ColumnInfo.

CloneCols uses (*Column).Clone to clone a slice of *Column.

CloneExprs uses Expression.Clone to clone a slice of Expression.

CloneRanges uses (*Range).Clone to clone a slice of *Range.

CompareCol2Len will compare the two Col2Len maps.

ExtractCol2Len collects index/table columns with lengths from expressions.

AccessPath indicates the way we access a table: by using single index, or by using multiple indexes, or just by using table scan.

ByItems wraps a "by" item.

Col2Len maps expression.Column.UniqueID to column length.