[hotfix][table-planner] Use equals instead of '==' to compare digests in FlinkCalcMergeRule

[lincoln-lil]

What is the purpose of the change

This is a trivial fix for string comparison in FlinkCalcMergeRule.
However it doesn't actually affect the optimization results, so no new cases were added to the fix.
But we can verify this detail by debugging FlinkCalcMergeRule's optimization process with the following case(add to FlinkCalcMergeRuleTest):

@Test
def testCalcMergeWithTrivialCalc(): Unit = {
  val testTable = "MyTable"
  val relBuilder = util.getPlanner.plannerContext.createRelBuilder()
  val flinkLogicalTraits = relBuilder.getCluster.traitSetOf(FlinkConventions.LOGICAL)
  val table = relBuilder.getRelOptSchema
    .asInstanceOf[CalciteCatalogReader]
    .getTable(ImmutableList.of(testTable))
    .asInstanceOf[FlinkPreparingTableBase]
  val flinkLogicalTableScan = new FlinkLogicalDataStreamTableScan(
    relBuilder.getCluster,
    flinkLogicalTraits,
    Collections.emptyList[RelHint](),
    table)

  relBuilder.scan(testTable)
  val logicScan = relBuilder.peek(0)
  val rowType = logicScan.getRowType
  val projects = (0 until (rowType.getFieldCount)).map(f => relBuilder.field(f)).toList
  val program = RexProgram.create(
    rowType,
    projects,
    null,
    rowType,
    relBuilder.getRexBuilder
  )

  val bottomCalc = FlinkLogicalCalc.create(flinkLogicalTableScan, program)
  // create a trivial calc
  val topCalc = FlinkLogicalCalc.create(bottomCalc, program)

  val optimizedRels = util.getPlanner.optimize(Array(topCalc))
  print(bottomCalc.getRelDigest, optimizedRels.get(0).getRelDigest)
  assertThat(bottomCalc.getRelDigest).isEqualTo(optimizedRels.get(0).getRelDigest)
  //    util.assertPlanEquals(Array(topCalc), Array.empty[ExplainDetail], withRowType = false, Array(PlanKind.OPT_REL), () => Unit)
}

Brief change log

fix string comparison

Verifying this change

existing tests

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @public(Evolving): (no)
• The serializers: (no )
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)

[flinkbot]

CI report:

• dcaa983 Azure: SUCCESS

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[lincoln-lil]

@snuyanzin Could you help look at this minor fix?

[snuyanzin]

should we have a backport to 2.0?

[lincoln-lil]

should we have a backport to 2.0?

I think so, will create a backport pr soon.

[lincoln-lil]

@snuyanzin Thanks for reviewing this! Backport pr #26110 to 2.0 branch has been created.