siddharthteotia commented on code in PR #8398:
URL: https://github.com/apache/pinot/pull/8398#discussion_r842448677
##########
pinot-segment-local/src/main/java/org/apache/pinot/segment/local/segment/creator/impl/SegmentColumnarIndexCreator.java:
##########
@@ -260,6 +260,26 @@ private boolean
createDictionaryForColumn(ColumnIndexCreationInfo info, SegmentG
.containsKey(column)) {
return false;
}
+
+ // Do not create dictionary if size with dictionary is going to be larger
than size without dictionary
+ // This is done to reduce the cost of dictionary for high cardinality
columns
+ // Off by default and needs optimizeDictionaryEnabled to be set to true
+ if (config.isOptimizeDictionaryEnabled() && spec.getFieldType() ==
FieldType.METRIC
+ && spec.isSingleValueField() && spec.getDataType().isFixedWidth()) {
+ long dictionarySize = info.getDistinctValueCount() *
spec.getDataType().size();
+ long forwardIndexSize =
+ ((long) info.getTotalNumberOfEntries() *
PinotDataBitSet.getNumBitsPerValue(info.getDistinctValueCount() - 1)
+ + Byte.SIZE - 1) / Byte.SIZE;
+
+ double indexWithDictSize = dictionarySize + forwardIndexSize;
+ double indexWithoutDictSize = info.getTotalNumberOfEntries() *
spec.getDataType().size();
+
+ double storageSaved = (indexWithDictSize - indexWithoutDictSize) /
indexWithDictSize;
Review Comment:
We can instead do a check based on ratio ?
`indexWithDictSize / indexWithoutDictSize`
Also, if we want to use percentage, then the reverse calculation will be
more intuitive imo as we want to compute the storage saved by creating
dictionary over no-dictionary
`double storageSaved = (indexWithoutDictSize - indexWithDictSize) /
indexWithoutDictSize;`
e.g -- long column with 1million rows and 16 cardinality
(8 * 1024 * 1024) - (128 + 500000) / (8 * 1024 * 1024) = 0.94 or 94%
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