[GitHub] [jena] afs commented on a diff in pull request #1631: GH-1615: LATERAL join

[GitBox <gi...@apache.org>]

afs commented on code in PR #1631:
URL: https://github.com/apache/jena/pull/1631#discussion_r1029650336


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jena-arq/src/main/java/org/apache/jena/sparql/algebra/optimize/TransformFilterImplicitJoin.java:
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@@ -120,10 +120,14 @@ private static Op apply(ExprList exprs, Op subOp) {
         // ---- Check if the subOp is the right shape to transform.
         Op op = subOp;
 
-        // Special case : deduce that the filter will always "eval unbound"
-        // hence eliminate all rows. Return the empty table.
+        // LATERAL : This is not longer true.
+//        // Special case : deduce that the filter will always "eval unbound"
+//        // hence eliminate all rows. Return the empty table.
+//        if (testSpecialCaseUnused(subOp, joins, remaining))
+//            return OpTable.empty();
+        // But simply skipping this causes (filter) to become (assign) which fails as (assign) does not handle errors.
         if (testSpecialCaseUnused(subOp, joins, remaining))
-            return OpTable.empty();
+            return null;

Review Comment:
   That's a possibility - I think it seems to designing for handling a bad query case (if not LATERAL), rather than optimization in LATERAL.
   
   It does seem to be a risk more widely, now and in future changes and not just in LATERAL - e.g. optimizing then substituting (e.g. parameterized queries). (NOT)EXISTS as well which is related to substitution. The fix for JENA-500 is a specific fix for the whole-query case.
   
   The transform walk is bottom up so it will run over the code - the walker needs to be changed to avoid certain ops.
   



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