Revert Combine small pages in Limit (#129107)

This PR reverts #128531.

With #128531, the Limit operator was updated to combine smaller pages 
into a larger page to reduce overhead, such as the number of exchange 
requests. However, this has a significant implication: the combined
larger page does not retain the attributes of the blocks from the
smaller pages. For example, if the smaller pages have ordinal-based
BytesRef blocks, the larger page will not. This can cause a significant
slowdown if subsequent operators have optimizations for ordinal-based
blocks. The Enrich operator has such optimizations, and our benchmarks
have shown this performance regression.

One possible solution to reduce the regression is to set a threshold 
(e.g., 1000 rows), above which the Limit operator would pass the page
along without combining. However, even with a threshold of 1000, the
performance regression does not go away completely. Alternatively, we
could allow exchange requests to return multiple pages (up to the page
size limit). To minimize risk, this PR reverts the previous change, and
we will reintroduce a new change later

Author: dnhatn

docs/changelog/128531.yaml

@@ -14,20 +14,14 @@
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
 import org.elasticsearch.compute.data.Block;
-import org.elasticsearch.compute.data.BlockFactory;
-import org.elasticsearch.compute.data.ElementType;
 import org.elasticsearch.compute.data.Page;
 import org.elasticsearch.core.Releasables;
 import org.elasticsearch.xcontent.XContentBuilder;
 
 import java.io.IOException;
-import java.util.ArrayList;
-import java.util.List;
 import java.util.Objects;
 
 public class LimitOperator implements Operator {
-    private final BlockFactory blockFactory;
-    private final int pageSize;
 
     /**
      * Count of pages that have been processed by this operator.
@@ -44,30 +38,25 @@ public class LimitOperator implements Operator {
      */
     private long rowsEmitted;
 
-    private final Limiter limiter;
+    private Page lastInput;
 
-    private final List<Page> queue = new ArrayList<>();
-    private int pendingRows;
+    private final Limiter limiter;
     private boolean finished;
 
-    public LimitOperator(Limiter limiter, BlockFactory blockFactory, int pageSize) {
+    public LimitOperator(Limiter limiter) {
         this.limiter = limiter;
-        this.blockFactory = blockFactory;
-        this.pageSize = pageSize;
     }
 
     public static final class Factory implements OperatorFactory {
         private final Limiter limiter;
-        private final int pageSize;
 
-        public Factory(int limit, int pageSize) {
+        public Factory(int limit) {
             this.limiter = new Limiter(limit);
-            this.pageSize = pageSize;
         }
 
         @Override
         public LimitOperator get(DriverContext driverContext) {
-            return new LimitOperator(limiter, driverContext.blockFactory(), pageSize);
+            return new LimitOperator(limiter);
         }
 
         @Override
@@ -78,20 +67,22 @@ public String describe() {
 
     @Override
     public boolean needsInput() {
-        return readyToEmit() == false;
+        return finished == false && lastInput == null && limiter.remaining() > 0;
     }
 
     @Override
     public void addInput(Page page) {
-        pagesProcessed++;
-        rowsReceived += page.getPositionCount();
+        assert lastInput == null : "has pending input page";
         final int acceptedRows = limiter.tryAccumulateHits(page.getPositionCount());
         if (acceptedRows == 0) {
             page.releaseBlocks();
+            assert isFinished();
+        } else if (acceptedRows < page.getPositionCount()) {
+            lastInput = truncatePage(page, acceptedRows);
         } else {
-            queue.add(page);
-            pendingRows += acceptedRows;
+            lastInput = page;
         }
+        rowsReceived += acceptedRows;
     }
 
     @Override
@@ -101,67 +92,41 @@ public void finish() {
 
     @Override
     public boolean isFinished() {
-        return pendingRows == 0 && (finished || limiter.remaining() == 0);
-    }
-
-    private boolean readyToEmit() {
-        return finished || pendingRows >= pageSize || limiter.remaining() == 0;
+        return lastInput == null && (finished || limiter.remaining() == 0);
     }
 
     @Override
     public Page getOutput() {
-        if (pendingRows > 0 && readyToEmit()) {
-            final Page result = combinePages(queue, blockFactory, pendingRows);
-            pendingRows = 0;
-            rowsEmitted += result.getPositionCount();
-            return result;
-        } else {
+        if (lastInput == null) {
             return null;
         }
+        final Page result = lastInput;
+        lastInput = null;
+        pagesProcessed++;
+        rowsEmitted += result.getPositionCount();
+        return result;
     }
 
-    private static ElementType[] elementTypes(int blockCount, List<Page> pages) {
-        ElementType[] elementTypes = new ElementType[blockCount];
-        for (Page page : pages) {
-            for (int b = 0; b < blockCount; b++) {
-                ElementType newType = page.getBlock(b).elementType();
-                ElementType currType = elementTypes[b];
-                if (currType == null || currType == ElementType.NULL) {
-                    elementTypes[b] = newType;
-                } else {
-                    assert newType == ElementType.NULL || currType == newType : "element type mismatch: " + currType + " != " + newType;
-                }
-            }
-        }
-        return elementTypes;
-    }
-
-    private static Page combinePages(List<Page> pages, BlockFactory blockFactory, int upTo) {
-        assert pages.isEmpty() == false : "no pages to combine";
-        if (pages.size() == 1 && pages.getFirst().getPositionCount() == upTo) {
-            return pages.removeFirst();
+    private static Page truncatePage(Page page, int upTo) {
+        int[] filter = new int[upTo];
+        for (int i = 0; i < upTo; i++) {
+            filter[i] = i;
         }
-        int blockCount = pages.getFirst().getBlockCount();
-        Block.Builder[] builders = new Block.Builder[blockCount];
+        final Block[] blocks = new Block[page.getBlockCount()];
+        Page result = null;
         try {
-            ElementType[] elementTypes = elementTypes(blockCount, pages);
-            for (int b = 0; b < blockCount; b++) {
-                builders[b] = elementTypes[b].newBlockBuilder(upTo, blockFactory);
-            }
-            int accumulated = 0;
-            for (Page page : pages) {
-                int size = Math.min(page.getPositionCount(), upTo - accumulated);
-                for (int b = 0; b < blockCount; b++) {
-                    Block block = page.getBlock(b);
-                    builders[b].copyFrom(block, 0, size);
-                }
-                accumulated += size;
+            for (int b = 0; b < blocks.length; b++) {
+                blocks[b] = page.getBlock(b).filter(filter);
             }
-            Block[] blocks = Block.Builder.buildAll(builders);
-            return new Page(blocks);
+            result = new Page(blocks);
         } finally {
-            Releasables.close(Releasables.wrap(pages), pages::clear, Releasables.wrap(builders));
+            if (result == null) {
+                Releasables.closeExpectNoException(page::releaseBlocks, Releasables.wrap(blocks));
+            } else {
+                page.releaseBlocks();
+            }
         }
+        return result;
     }
 
     @Override
@@ -171,7 +136,9 @@ public Status status() {
 
     @Override
     public void close() {
-        Releasables.close(queue);
+        if (lastInput != null) {
+            lastInput.releaseBlocks();
+        }
     }
 
     @Override

x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/operator/LimitOperator.java

@@ -142,7 +142,7 @@ public void doClose() {
         if (randomBoolean()) {
             int limit = between(0, ids.size());
             it = ids.subList(0, limit).iterator();
-            intermediateOperators.add(new LimitOperator(new Limiter(limit), blockFactory(), between(1, 1024)));
+            intermediateOperators.add(new LimitOperator(new Limiter(limit)));
         } else {
             it = ids.iterator();
         }

x-pack/plugin/esql/compute/src/test/java/org/elasticsearch/compute/operator/AsyncOperatorTests.java

@@ -9,14 +9,12 @@
 
 import org.elasticsearch.compute.data.Block;
 import org.elasticsearch.compute.data.BlockFactory;
-import org.elasticsearch.compute.data.ElementType;
 import org.elasticsearch.compute.data.Page;
 import org.elasticsearch.compute.test.OperatorTestCase;
 import org.elasticsearch.compute.test.RandomBlock;
 import org.elasticsearch.compute.test.SequenceLongBlockSourceOperator;
 import org.elasticsearch.core.TimeValue;
 import org.hamcrest.Matcher;
-import org.junit.Before;
 
 import java.util.ArrayList;
 import java.util.List;
@@ -30,7 +28,7 @@
 public class LimitOperatorTests extends OperatorTestCase {
     @Override
     protected LimitOperator.Factory simple(SimpleOptions options) {
-        return new LimitOperator.Factory(100, 500);
+        return new LimitOperator.Factory(100);
     }
 
     @Override
@@ -48,30 +46,6 @@ protected Matcher<String> expectedToStringOfSimple() {
         return equalTo("LimitOperator[limit = 100/100]");
     }
 
-    private ElementType elementType;
-
-    @Before
-    public void setUpElementTypes() throws Exception {
-        elementType = randomFrom(ElementType.INT, ElementType.NULL, ElementType.BYTES_REF);
-    }
-
-    private Page randomPage(BlockFactory blockFactory, int size) {
-        if (randomBoolean()) {
-            return new Page(blockFactory.newConstantNullBlock(size));
-        }
-        Block block = RandomBlock.randomBlock(
-            blockFactory,
-            elementType,
-            size,
-            elementType == ElementType.NULL || randomBoolean(),
-            1,
-            1,
-            0,
-            0
-        ).block();
-        return new Page(block);
-    }
-
     @Override
     protected void assertSimpleOutput(List<Page> input, List<Page> results) {
         int inputPositionCount = input.stream().mapToInt(p -> p.getPositionCount()).sum();
@@ -91,7 +65,6 @@ public void testStatus() {
         Page p = new Page(blockFactory.newConstantNullBlock(10));
         try {
             op.addInput(p);
-            op.finish();
             assertSame(p, op.getOutput());
         } finally {
             p.releaseBlocks();
@@ -104,38 +77,24 @@ public void testStatus() {
 
     public void testNeedInput() {
         BlockFactory blockFactory = driverContext().blockFactory();
-        // small page size
-        try (LimitOperator op = new LimitOperator(new Limiter(100), blockFactory, 5)) {
+        try (LimitOperator op = simple(SimpleOptions.DEFAULT).get(driverContext())) {
             assertTrue(op.needsInput());
-            Page p = randomPage(blockFactory, 10);
+            Page p = new Page(blockFactory.newConstantNullBlock(10));
             op.addInput(p);
             assertFalse(op.needsInput());
             op.getOutput().releaseBlocks();
             assertTrue(op.needsInput());
             op.finish();
             assertFalse(op.needsInput());
         }
-        // small page size
-        try (LimitOperator op = new LimitOperator(new Limiter(100), blockFactory, 50)) {
-            for (int i = 0; i < 5; i++) {
-                assertTrue(op.needsInput());
-                Page p = randomPage(blockFactory, 10);
-                op.addInput(p);
-            }
-            assertFalse(op.needsInput());
-            op.getOutput().releaseBlocks();
-            assertTrue(op.needsInput());
-            op.finish();
-            assertFalse(op.needsInput());
-        }
     }
 
     public void testBlockBiggerThanRemaining() {
         BlockFactory blockFactory = driverContext().blockFactory();
         for (int i = 0; i < 100; i++) {
             try (var op = simple().get(driverContext())) {
                 assertTrue(op.needsInput());
-                Page p = randomPage(blockFactory, 200);  // test doesn't close because operator returns a view
+                Page p = new Page(randomBlock(blockFactory, 200));  // test doesn't close because operator returns a view
                 op.addInput(p);
                 assertFalse(op.needsInput());
                 Page result = op.getOutput();
@@ -155,7 +114,7 @@ public void testBlockPreciselyRemaining() {
         for (int i = 0; i < 100; i++) {
             try (var op = simple().get(driverContext())) {
                 assertTrue(op.needsInput());
-                Page p = randomPage(blockFactory, 100);  // test doesn't close because operator returns same page
+                Page p = new Page(randomBlock(blockFactory, 100));  // test doesn't close because operator returns same page
                 op.addInput(p);
                 assertFalse(op.needsInput());
                 Page result = op.getOutput();
@@ -174,7 +133,7 @@ public void testEarlyTermination() {
         int numDrivers = between(1, 4);
         final List<Driver> drivers = new ArrayList<>();
         final int limit = between(1, 10_000);
-        final LimitOperator.Factory limitFactory = new LimitOperator.Factory(limit, between(1024, 2048));
+        final LimitOperator.Factory limitFactory = new LimitOperator.Factory(limit);
         final AtomicInteger receivedRows = new AtomicInteger();
         for (int i = 0; i < numDrivers; i++) {
             DriverContext driverContext = driverContext();
@@ -193,8 +152,7 @@ public boolean isFinished() {
 
                 @Override
                 public Page getOutput() {
-                    return randomPage(blockFactory(), between(1, 100));
-
+                    return new Page(randomBlock(driverContext.blockFactory(), between(1, 100)));
                 }
 
                 @Override

x-pack/plugin/esql/compute/src/test/java/org/elasticsearch/compute/operator/LimitOperatorTests.java

@@ -19,10 +19,7 @@
 import org.elasticsearch.common.util.concurrent.AbstractRunnable;
 import org.elasticsearch.common.util.concurrent.EsRejectedExecutionException;
 import org.elasticsearch.common.util.iterable.Iterables;
-import org.elasticsearch.compute.operator.DriverProfile;
-import org.elasticsearch.compute.operator.OperatorStatus;
 import org.elasticsearch.compute.operator.exchange.ExchangeService;
-import org.elasticsearch.compute.operator.exchange.ExchangeSinkOperator;
 import org.elasticsearch.core.TimeValue;
 import org.elasticsearch.plugins.Plugin;
 import org.elasticsearch.rest.RestStatus;
@@ -49,7 +46,6 @@
 import java.util.concurrent.atomic.AtomicReference;
 
 import static org.hamcrest.Matchers.equalTo;
-import static org.hamcrest.Matchers.hasSize;
 import static org.hamcrest.Matchers.instanceOf;
 import static org.hamcrest.Matchers.lessThanOrEqualTo;
 
@@ -212,34 +208,6 @@ public void sendResponse(Exception exception) {
         }
     }
 
-    public void testLimitCombineSmallerPages() {
-        QueryPragmas queryPragmas = randomPragmas();
-        if (canUseQueryPragmas()) {
-            Settings.Builder settings = Settings.builder().put(queryPragmas.getSettings());
-            settings.remove(QueryPragmas.NODE_LEVEL_REDUCTION.getKey());
-            settings.remove(QueryPragmas.PAGE_SIZE.getKey());
-            queryPragmas = new QueryPragmas(settings.build());
-        }
-        var request = new EsqlQueryRequest();
-        request.query("FROM test-* | KEEP user | LIMIT 100");
-        request.pragmas(queryPragmas);
-        request.profile(true);
-        try (EsqlQueryResponse resp = run(request)) {
-            List<DriverProfile> nodeReduce = resp.profile().drivers().stream().filter(s -> s.description().equals("node_reduce")).toList();
-            for (DriverProfile driverProfile : nodeReduce) {
-                if (driverProfile.operators().size() == 2) {
-                    continue; // when the target node is also the coordinator node
-                }
-                assertThat(driverProfile.operators(), hasSize(3));
-                OperatorStatus exchangeSink = driverProfile.operators().get(2);
-                assertThat(exchangeSink.status(), instanceOf(ExchangeSinkOperator.Status.class));
-                ExchangeSinkOperator.Status exchangeStatus = (ExchangeSinkOperator.Status) exchangeSink.status();
-                assertThat(exchangeStatus.pagesReceived(), lessThanOrEqualTo(1));
-            }
-            assertThat(resp.pages(), hasSize(1));
-        }
-    }
-
     static class SearchContextCounter {
         private final int maxAllowed;
         private final AtomicInteger current = new AtomicInteger();

x-pack/plugin/esql/src/internalClusterTest/java/org/elasticsearch/xpack/esql/action/ManyShardsIT.java

@@ -837,10 +837,7 @@ private PhysicalOperation planFilter(FilterExec filter, LocalExecutionPlannerCon
 
     private PhysicalOperation planLimit(LimitExec limit, LocalExecutionPlannerContext context) {
         PhysicalOperation source = plan(limit.child(), context);
-        final Integer rowSize = limit.estimatedRowSize();
-        assert rowSize != null && rowSize > 0 : "estimated row size [" + rowSize + "] wasn't set";
-        int pageSize = context.pageSize(rowSize);
-        return source.with(new LimitOperator.Factory((Integer) limit.limit().fold(context.foldCtx), pageSize), source.layout);
+        return source.with(new LimitOperator.Factory((Integer) limit.limit().fold(context.foldCtx)), source.layout);
     }
 
     private PhysicalOperation planMvExpand(MvExpandExec mvExpandExec, LocalExecutionPlannerContext context) {