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[core] Reject a merge fan-in below two up front #10293

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[core] Reject a merge fan-in below two up front #10293

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9 changes: 9 additions & 0 deletions paimon-core/src/main/java/org/apache/paimon/sort/AbstractBinaryExternalMerger.java
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Original file line number Diff line number Diff line change
Expand Up @@ -18,6 +18,7 @@

package org.apache.paimon.sort;

import org.apache.paimon.CoreOptions;
import org.apache.paimon.compression.BlockCompressionFactory;
import org.apache.paimon.data.AbstractPagedOutputView;
import org.apache.paimon.disk.ChannelReaderInputView;
Expand All @@ -37,6 +38,8 @@
import java.util.Comparator;
import java.util.List;

import static org.apache.paimon.utils.Preconditions.checkArgument;

/**
* Spilled files Merger of {@link BinaryExternalSortBuffer}. It merges {@link #maxFanIn} spilled
* files at most once.
Expand Down Expand Up @@ -64,6 +67,12 @@ public AbstractBinaryExternalMerger(
SpillChannelManager channelManager,
BlockCompressionFactory compressionCodecFactory,
int compressionBlockSize) {
checkArgument(
maxFanIn >= 2,
"The fan-in for external merge sort must be at least 2, but was %s. "
+ "Please adjust '%s'.",
maxFanIn,
CoreOptions.LOCAL_SORT_MAX_NUM_FILE_HANDLES.key());
this.ioManager = ioManager;
this.pageSize = pageSize;
this.maxFanIn = maxFanIn;
Expand Down
9 changes: 9 additions & 0 deletions paimon-core/src/test/java/org/apache/paimon/sort/BinaryExternalSortBufferTest.java
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Expand Up @@ -428,4 +428,13 @@ public BinaryRow next() {
return row;
}
}

@Test
void testFanInBelowTwoFailsFast() {
// a fan-in of 1 previously overflowed the merge arithmetic at flush time and
// restart-looped on the same config; it must be rejected up front
assertThatThrownBy(() -> createBuffer(1))
.isInstanceOf(IllegalArgumentException.class)
.hasMessageContaining("fan-in for external merge sort must be at least 2");
}
}
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