[CALCITE-3786] Add Digest interface to enable efficient hashCode(equals) for RexNode and RelNode

• Type: New Feature
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 1.21.0
• Fix Version/s: 1.24.0
• Component/s: core
• Assignee: Danny Chen
• Reporter: Vladimir Sitnikov
• Created: 12/Feb/20 11:31
• Updated: 25/Jul/20 14:59
• Resolved: 24/Jun/20 16:12

Description

Current digests for RexNode, RelNode, RelType, and similar cases use String concatenation.

It is easy to implement, however, it has drawbacks:
1) String objects cannot be reused. For instance, RexCall has operands, however, the digest is duplicated. It causes extra memory use and extra CPU for string copying
2) There's no way to have multiple #toString() methods. RelType might need multiple digests: "including field names", "excluding field names".

A suggested resolution might be behind the lines of

class Digest { // immutable
  final int hashCode; // speedup hashCode and equals
  final Object[] contents; // The values are either other Digest objects or Strings
  String toString(); // e.g. for debugging purposes
  int compareTo(Digest); // e.g. for debugging purposes.
}

Note how fields in Kotlin are aligned much better, and it makes it easier to read:

class Digest { // immutable
  val hashCode: Int // speedup hashCode and equals
  val contents: Array<Any> // The values are either other Digest objects or Strings
  fun toString(): String // e.g. for debugging purposes
  fun compareTo(other: Digest): Int // e.g. for debugging purposes.
}

Then the digest for RexCall could be the bits relevant to RexCall itself + digests of the operands (which can be reused as is)

Sub-Tasks

• Add benchmark test for new Digest interface: Open