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https://issues.apache.org/jira/browse/CALCITE-4223?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17192359#comment-17192359
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Julian Hyde commented on CALCITE-4223:
--------------------------------------
I don't think we should extend {{interface RelOptTable}} or {{interface
Statistics}}. There are a few reasons.
First, {{RelOptTable}} already extends {{interface Wrapper}}. If you want your
implementation of {{RelOptTable}} to provide some other interface, you can get
it via {{Wrapper.unwrap()}}.
Second, there is no fixed definition of statistics. Different engines are going
to have different variations. One engine might have a histogram that tells you
the number of distinct values of productId when state = 'CA'; another might
not. That's why we created the {{RelMetadataQuery}} framework.
Third and fourth, we will need caching, and a way to plug in multiple
providers. Again, that's what the {{RelMetadataQuery}} framework is for.
Why not write a convenience method
{code}
double getNdv(RelMetadataQuery mq, RelOptTable table, int column) {
return mq.getPopulationSize(new TableScan(table),
ImmutableBitSet.of(column));
{code}
> Introducing column statistics to RelOptTable
> --------------------------------------------
>
> Key: CALCITE-4223
> URL: https://issues.apache.org/jira/browse/CALCITE-4223
> Project: Calcite
> Issue Type: Improvement
> Reporter: Chunwei Lei
> Assignee: Chunwei Lei
> Priority: Major
>
> Many systems depend on column statistics to compute more accurate stats, such
> as NDV, average column size, and so on. It would be nice if Calcite can
> provide such an interface.
> Column statistics might include NDV, average/max column length, number of
> nulls, number of trues, number of falses and so on.
> What do you think?
>
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