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Posted to commits@druid.apache.org by GitBox <gi...@apache.org> on 2021/08/04 21:49:59 UTC
[GitHub] [druid] paul-rogers commented on a change in pull request #11549: Docs: Clarify segmentMetadata cardinality, minmax, and size behavior.
paul-rogers commented on a change in pull request #11549:
URL: https://github.com/apache/druid/pull/11549#discussion_r682983402
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File path: docs/querying/segmentmetadataquery.md
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@@ -144,16 +144,16 @@ Types of column analyses are described below:
### cardinality
-* `cardinality` in the result will return the size of the bitmap index or dictionary encoding for string dimensions, or null for other dimension types.
- If `merge` was set, the result will be the max of this value across segments. Only relevant for dimension columns.
+* `cardinality` in the result will return the number of unique values present in a string column. It is null for other column types.
+ If `merge` is set, the result will be the max of this value across segments. Only relevant for string columns.
### minmax
-* Estimated min/max values for each column. Only relevant for dimension columns.
+* Estimated min/max values for each column. Only reported for string columns.
### size
-* `size` in the result will contain the estimated total segment byte size as if the data were stored in text format
+* `size` in the result will contain the estimated total segment byte size as if the data were stored in text format. This is _not_ the actual storage size of the column in Druid.
Review comment:
Pointer to where I might find the actual storage size? I want to know the amount of space the column takes so I know if it is worth the cost to store.
##########
File path: docs/querying/segmentmetadataquery.md
##########
@@ -144,16 +144,16 @@ Types of column analyses are described below:
### cardinality
-* `cardinality` in the result will return the size of the bitmap index or dictionary encoding for string dimensions, or null for other dimension types.
- If `merge` was set, the result will be the max of this value across segments. Only relevant for dimension columns.
+* `cardinality` in the result will return the number of unique values present in a string column. It is null for other column types.
+ If `merge` is set, the result will be the max of this value across segments. Only relevant for string columns.
Review comment:
This is not clear to us newbies. Does "max" mean the largest number of any segment, or the aggregated total across segments? Both are useful: if I have 1M rows, and see a cardinality of 1K, that could mean 1K total, or 1K per segment, which says something else if each segment has 1K rows...
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