Enhance optimizeDictionary to optionally optimize var-width type cols
#13994
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## master #13994 +/- ##
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+ Coverage 61.75% 65.09% +3.34%
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SabrinaZhaozyf
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Sep 14, 2024
Jackie-Jiang
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Configuration
...c/main/java/org/apache/pinot/segment/local/segment/index/dictionary/DictionaryIndexType.java
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| @@ -293,8 +293,9 @@ private boolean createDictionaryForColumn(ColumnIndexCreationInfo info, SegmentG | |||
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| FieldIndexConfigs fieldIndexConfigs = config.getIndexConfigsByColName().get(column); | |||
| if (DictionaryIndexType.ignoreDictionaryOverride(config.isOptimizeDictionary(), | |||
| config.isOptimizeDictionaryForMetrics(), config.getNoDictionarySizeRatioThreshold(), spec, fieldIndexConfigs, | |||
| info.getDistinctValueCount(), info.getTotalNumberOfEntries())) { | |||
| config.isOptimizeDictionaryForMetrics(), config.getNoDictionarySizeRatioThreshold(), | |||
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This list has become quite long. Suggest adding a new config class for dictionary tuning related configs
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Changes
noDictionaryCardinalityRatioThresholdconfig.optimizeDictionaryis true, then Pinot will override dictionary encoding with raw encoding based on the conditioncardinality / numDocs > noDictionaryCardinalityRatioThreshold.optimizeDictionarybehavior is unchangedMotivation
When storing log data, often columns will contain many repeated values. It's useful to take advantage of Pinot's dictionary encoding which usually provides better storage/query performance for these columns. Dictionary encoding high cardinality columns is cost/storage prohibitive, so we'd like to avoid applying dictionary encoding unless it is safe. Since column cardinality/values can change rapidly we'd like to make these decisions within Pinot itself.
In our experience, cardinality is a good indicator of whether to dictionary or raw encode a col. With a
0.10threshold (10%), we see roughly 40-60% improvement in storage compared to raw encoding everything.