Fix the lazy block prefilter (#1657)

This fixes a bug introduced in #1647, which led to a failure of ` AD_CORRECTNESS_CHECK(!prefetchedValues.empty() || innerState->doneFetching_);` in `IndexScan::createPrefilteredJoinSide`.
ad-freiburg · Dec 4, 2024 · ec806f0 · ec806f0
1 parent 23f8075
commit ec806f0
Show file tree

Hide file tree

Showing 2 changed files with 37 additions and 31 deletions.
diff --git a/src/engine/IndexScan.cpp b/src/engine/IndexScan.cpp
@@ -532,39 +532,43 @@ struct IndexScan::SharedGeneratorState {
   }
 
   // Consume the next non-empty table from the generator and calculate the next
-  // matching blocks from the index scan.
+  // matching blocks from the index scan. This function guarantees that after
+  // it returns, both `prefetchedValues` and `pendingBlocks` contain at least
+  // one element.
   void fetch() {
-    advanceInputToNextNonEmptyTable();
-    if (doneFetching_) {
-      return;
-    }
-    auto& idTable = iterator_.value()->idTable_;
-    auto joinColumn = idTable.getColumn(joinColumn_);
-    AD_EXPENSIVE_CHECK(std::ranges::is_sorted(joinColumn));
-    AD_CORRECTNESS_CHECK(!joinColumn.empty());
-    // Skip processing for undef case, it will be handled differently
-    if (hasUndef_) {
-      return;
-    }
-    AD_CORRECTNESS_CHECK(!joinColumn[0].isUndefined());
-    auto newBlocks =
-        CompressedRelationReader::getBlocksForJoin(joinColumn, metaBlocks_);
-    if (newBlocks.empty()) {
-      // The current input table matches no blocks, so we don't have to yield
-      // it.
-      return;
+    while (prefetchedValues_.empty() || pendingBlocks_.empty()) {
+      advanceInputToNextNonEmptyTable();
+      if (doneFetching_) {
+        return;
+      }
+      auto& idTable = iterator_.value()->idTable_;
+      auto joinColumn = idTable.getColumn(joinColumn_);
+      AD_EXPENSIVE_CHECK(std::ranges::is_sorted(joinColumn));
+      AD_CORRECTNESS_CHECK(!joinColumn.empty());
+      // Skip processing for undef case, it will be handled differently
+      if (hasUndef_) {
+        return;
+      }
+      AD_CORRECTNESS_CHECK(!joinColumn[0].isUndefined());
+      auto newBlocks =
+          CompressedRelationReader::getBlocksForJoin(joinColumn, metaBlocks_);
+      if (newBlocks.empty()) {
+        // The current input table matches no blocks, so we don't have to yield
+        // it.
+        continue;
+      }
+      prefetchedValues_.push_back(std::move(*iterator_.value()));
+      // Find first value that differs from the last one that was used to find
+      // matching blocks.
+      auto startIterator =
+          lastBlockIndex_.has_value()
+              ? std::ranges::upper_bound(newBlocks, lastBlockIndex_.value(), {},
+                                         &CompressedBlockMetadata::blockIndex_)
+              : newBlocks.begin();
+      lastBlockIndex_ = newBlocks.back().blockIndex_;
+      std::ranges::move(startIterator, newBlocks.end(),
+                        std::back_inserter(pendingBlocks_));
     }
-    prefetchedValues_.push_back(std::move(*iterator_.value()));
-    // Find first value that differs from the last one that was used to find
-    // matching blocks.
-    auto startIterator =
-        lastBlockIndex_.has_value()
-            ? std::ranges::upper_bound(newBlocks, lastBlockIndex_.value(), {},
-                                       &CompressedBlockMetadata::blockIndex_)
-            : newBlocks.begin();
-    lastBlockIndex_ = newBlocks.back().blockIndex_;
-    std::ranges::move(startIterator, newBlocks.end(),
-                      std::back_inserter(pendingBlocks_));
   }
 
   // Check if there are any undefined values yielded by the original generator.

diff --git a/test/engine/IndexScanTest.cpp b/test/engine/IndexScanTest.cpp
@@ -939,6 +939,8 @@ TEST_P(IndexScanWithLazyJoin,
     using P = Result::IdTableVocabPair;
     P p1{makeIdTableFromVector({{Id::makeFromBool(true)}}), LocalVocab{}};
     co_yield p1;
+    P p2{makeIdTableFromVector({{Id::makeFromBool(true)}}), LocalVocab{}};
+    co_yield p2;
   };
 
   auto [joinSideResults, scanResults] =