[I] CometPlainVector: validity-bitmap byte cache for sequential reads [datafusion-comet]

["mbutrovich (via GitHub)" <gi...@apache.org>]

mbutrovich opened a new issue, #4279:
URL: https://github.com/apache/datafusion-comet/issues/4279

   ## What is the problem the feature request solves?
   
   `CometPlainVector.getBoolean(int)` already caches the last-read data-bitmap byte to avoid re-reading it eight times per byte of bit-packed booleans:
   
   ```java
   private byte booleanByteCache;
   private int booleanByteCacheIndex = -1;
   
   public boolean getBoolean(int rowId) {
     int byteIndex = rowId >> 3;
     if (byteIndex != booleanByteCacheIndex) {
       booleanByteCache = getByte(byteIndex);
       booleanByteCacheIndex = byteIndex;
     }
     return ((booleanByteCache >> (rowId & 7)) & 1) == 1;
   }
   ```
   
   The validity bitmap (checked by `isNullAt`) is also bit-packed one bit per row, but it has no equivalent cache. Sequential row reads on a nullable column hit the same validity-buffer byte eight times in a row. Every caller that reads a column row-by-row (including the native scan output path, shuffle read path, and the JVM UDF dispatch kernel in #4267) pays the duplicate reads.
   
   ## Describe the potential solution
   
   Add a sibling byte cache for the validity bitmap, using the same pattern already present for `booleanByteCache`:
   
   ```java
   private byte validityByteCache;
   private int validityByteCacheIndex = -1;
   
   @Override
   public boolean isNullAt(int rowId) {
     if (this.valueBufferAddress == -1) return true;
     int byteIndex = rowId >> 3;
     if (byteIndex != validityByteCacheIndex) {
       validityByteCache = /* read validity-buffer byte at byteIndex */;
       validityByteCacheIndex = byteIndex;
     }
     return ((validityByteCache >> (rowId & 7)) & 1) == 0;
   }
   
   @Override
   public void setNumNulls(int numNulls) {
     super.setNumNulls(numNulls);
     this.booleanByteCacheIndex = -1;
     this.validityByteCacheIndex = -1;  // invalidate on state transitions
   }
   ```
   
   Collapses the 8x duplicate validity-byte reads to one read per 8 rows on sequential access.
   
   ## Additional context
   
   - Mirrors the established pattern in the same class (`booleanByteCache`).
   - The existing `setNumNulls` override resets `booleanByteCacheIndex`; the validity cache would extend that.
   - No API change. No behavior change. Strictly reduces per-row work for any nullable column read sequentially.
   - Random-access patterns see no win but no regression either (cache miss on every call, same cost as today after the miss).
   


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Re: [I] CometPlainVector: validity-bitmap byte cache for sequential reads [datafusion-comet]

["mbutrovich (via GitHub)" <gi...@apache.org>]

mbutrovich closed issue #4279: CometPlainVector: validity-bitmap byte cache for sequential reads
URL: https://github.com/apache/datafusion-comet/issues/4279


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