[CALCITE-1498] Avoid LIMIT with trivial ORDER BY being pushed through JOIN endlessly

• Type: Bug
• Status: Closed
• Priority: Minor
• Resolution: Fixed
• Affects Version/s: 1.10.0
• Fix Version/s: 1.11.0
• Component/s: core
• Assignee: Wei Xue
• Reporter: Wei Xue
• Created: 21/Nov/16 00:38
• Updated: 06/Jan/17 19:29
• Resolved: 23/Nov/16 20:35

Description

Currently LIMIT with trivial ORDER BY will be pushed through a JOIN endlessly by SortJoinTransposeRule, because the method RelMdUtil.checkInputForCollationAndLimit used to prevent endless matching does not know that an sort on zero keys is trivially satisfied (without requiring a Sort) by any relational expression:

    // Check if the input is already sorted
    boolean alreadySorted = false;
    if (!alreadySorted) {
      for (RelCollation inputCollation : mq.collations(input)) {
        if (inputCollation.satisfies(collation)) {
          alreadySorted = true;
          break;
        }
      }
    }

if mq.collations(input) returns an empty array, alreadySorted will always be false even if the required collation is an empty collation (which indicates there's no need to sort). As a result, the check method RelMdUtil.checkInputForCollationAndLimit will always return false and the SortJoinTransposeRule will keep being fired endlessly.