1. /**
    2.  * A collection of expressions which read inputs, compute output expressions,
    3.  * and optionally use a condition to filter rows.
    4.  *
    5.  * <p>Programs are immutable. It may help to use a {@link RexProgramBuilder},
    6.  * which has the same relationship to {@link RexProgram} as {@link StringBuilder}
    7.  * has to {@link String}.
    8.  *
    9.  * <p>A program can contain aggregate functions. If it does, the arguments to
    10.  * each aggregate function must be an {@link RexInputRef}.
    11.  *
    12.  * @see RexProgramBuilder
    13.  */
    14. public class RexProgram {
    15.   //~ Instance fields --------------------------------------------------------
    17.   /**
    18.    * First stage of expression evaluation. The expressions in this array can
    19.    * refer to inputs (using input ordinal #0) or previous expressions in the
    20.    * array (using input ordinal #1).
    21.    */
    22.   // 最开始的表达式信息，一般是RexInputRef, RexCall, RexLocalRef 等node
    23.   private final List<RexNode> exprs;
    25.   /**
    26.    * With {@link #condition}, the second stage of expression evaluation.
    27.    */
    28.   // 经过 Condition 条件过滤之后的表达式信息
    29.   private final List<RexLocalRef> projects;
    31.   /**
    32.    * The optional condition. If null, the calculator does not filter rows.
    33.    */
    34.   private final RexLocalRef condition;
    36.   private final RelDataType inputRowType;
    38.   private final RelDataType outputRowType;
    40.   /**
    41.    * Reference counts for each expression, computed on demand.
    42.    */
    43.   private int[] refCounts;
    44. }