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Posted to github@datafusion.apache.org by "mbutrovich (via GitHub)" <gi...@apache.org> on 2026/05/10 02:33:42 UTC

[I] CometPlainVector: cache offsetBufferAddress for variable-width vectors [datafusion-comet]

mbutrovich opened a new issue, #4280:
URL: https://github.com/apache/datafusion-comet/issues/4280

   ## What is the problem the feature request solves?
   
   `CometPlainVector.getUTF8String(int)` and `CometPlainVector.getBinary(int)` dereference the offset buffer address on every call:
   
   ```java
   long offsetBufferAddress = varWidthVector.getOffsetBuffer().memoryAddress();
   int offset = Platform.getInt(null, offsetBufferAddress + rowId * 4L);
   ```
   
   That is one Arrow method call, one buffer dereference, and one `memoryAddress` call per row, duplicating work `valueBufferAddress` already avoids (the value buffer address is cached as a `final long` field in the constructor).
   
   For workloads that read VarChar or VarBinary columns heavily through `CometPlainVector` (native scan, shuffle, and the JVM UDF dispatch path introduced in #4267), the per-row overhead is pure bookkeeping.
   
   ## Describe the potential solution
   
   Hoist the offset buffer address into a `final long offsetBufferAddress` field set in the constructor alongside `valueBufferAddress`, and use it directly in `getUTF8String` / `getBinary`:
   
   ```java
   private final long offsetBufferAddress;
   
   public CometPlainVector(ValueVector vector, boolean useDecimal128, boolean isUuid, boolean isReused) {
     // ...
     if (vector instanceof BaseVariableWidthVector) {
       this.offsetBufferAddress = ((BaseVariableWidthVector) vector).getOffsetBuffer().memoryAddress();
     } else {
       this.offsetBufferAddress = -1;
     }
   }
   
   public UTF8String getUTF8String(int rowId) {
     if (isNullAt(rowId)) return null;
     if (!isBaseFixedWidthVector) {
       int offset = Platform.getInt(null, offsetBufferAddress + rowId * 4L);
       int length = Platform.getInt(null, offsetBufferAddress + (rowId + 1L) * 4L) - offset;
       return UTF8String.fromAddress(null, valueBufferAddress + offset, length);
     }
     // fixed-width branch unchanged
   }
   ```
   
   Same pattern for `getBinary`. JIT treats the address as a register-cached constant across the hot loop.
   
   ## Additional context
   
   - `valueBufferAddress` already uses this pattern; this proposal extends it for symmetry and for the same reason it was used there.
   - Arrow guarantees `getOffsetBuffer()` returns the same buffer object for the lifetime of the vector, so caching the address at construction is safe.
   - No API surface change. No behavior change. Strictly reduces per-row work for variable-width reads.
   - Benefits every current caller of `CometPlainVector`, not just one code path.
   


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Re: [I] CometPlainVector: cache offsetBufferAddress for variable-width vectors [datafusion-comet]

Posted by "0lai0 (via GitHub)" <gi...@apache.org>.
0lai0 commented on issue #4280:
URL: https://github.com/apache/datafusion-comet/issues/4280#issuecomment-4426843536

   take


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Re: [I] CometPlainVector: cache offsetBufferAddress for variable-width vectors [datafusion-comet]

Posted by "andygrove (via GitHub)" <gi...@apache.org>.
andygrove closed issue #4280: CometPlainVector: cache offsetBufferAddress for variable-width vectors
URL: https://github.com/apache/datafusion-comet/issues/4280


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