[GitHub] [drill] paul-rogers edited a comment on issue #1923: DRILL-7479: Partial fixes for metadata parameterized type issues

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paul-rogers edited a comment on issue #1923: DRILL-7479: Partial fixes for metadata parameterized type issues
URL: https://github.com/apache/drill/pull/1923#issuecomment-565141768
 
 
   Tried making  `ColumnStatistics` generic. However there are several places where the type of the column is required (min/max values, for example.)
   
   The issue is that each column has multiple stats: some are of the same type as the column (min, max, histogram), others are not (HLL, NDV.)
   
   Even those that are the "same" type as the column are not: the column is of type `VARCHAR`, but the `ColumnStatistics` is of type `String`, say. There is no mapping from Drill to Java types; this knowedge appears to be implicit in the code.
   
   Further, the `ColumnStatisticsKind` is not what you would think: it has a compile-time type (the type parameter), but, because of type erasure, no runtime type (I cannot ask what kind of value the kind holds.)
   
   I think we need to rethink this a bit:
   
   * A type definition holds the type-specific logic (such as merging stats) as well as giving both the Java and SQL type (if any) for the stat.
   * A mapping tells us the Java stats type for the SQL type.
   * A stats definition holds a name and a type definition. (Where the type definition may be "same as column type.")
   * A Statistic has a definition, a (possibly implied) type and a value.
   * A column is generic: it has a name, a SQL type and a list of statistics.
   * It may be worthwhile having a column "meta-meta" definition: the list of statistics valid for a column of a given SQL type. (There is no min or max for a VarBinary, say.)
   
   This PR at least removes many warnings an obscure compile error from the present design.

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