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https://issues.apache.org/jira/browse/CALCITE-4223?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17192601#comment-17192601
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Chunwei Lei commented on CALCITE-4223:
--------------------------------------
There is always an alternative due to the flexible interface. But I think it
would be great if Calcite has column statistics since it helps generate a
better plan.
Even if the column statistics are provided, we still get the statistics like
NDV using {{RelMetadataQuery}}. What we need to do is add/modify some methods
in RelMdxxx. Taking NDV for example:
{code:java}
// RelMdDistinctRowCount.java
public Double getDistinctRowCount(TableScan rel,
RelMetadataQuery mq, ImmutableBitSet groupKey,
RexNode predicate) {
......
List<Double> allDistinctValue =
getAllDistinctValue(rel.getTable().getColumnStatistics(), groupKey);
return getJointDistinctValue(allDistinctValue);
}{code}
Regarding no fixed definition of statistics, we can provide some basic and
frequently-used column statistics including NDV/AverageColumnSize/nullCount.
Let me conclude to help others understand more. There are two questions:
1) Do you agree to introduce column statistics?
2) If so, where should we put them? (RelOptTable? Statistics? Or other places)
> 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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