clintropolis commented on code in PR #17084:
URL: https://github.com/apache/druid/pull/17084#discussion_r1762645130
##########
processing/src/main/java/org/apache/druid/segment/VirtualColumns.java:
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@@ -172,10 +174,28 @@ public VirtualColumn getVirtualColumn(String columnName)
if (vc != null) {
return vc;
}
+ if (hasNoDotColumns) {
+ return null;
+ }
final String baseColumnName = splitColumnName(columnName).lhs;
return withDotSupport.get(baseColumnName);
}
+ /**
+ * Check if a virtual column is already defined which is the same as some
other virtual column, ignoring output name,
+ * returning that virtual column if it exists, or null if there is no
equivalent virtual column.
+ */
+ @Nullable
+ public VirtualColumn findEquivalent(VirtualColumn virtualColumn)
Review Comment:
perhaps the context of why I am adding this method is missing. I'm working
on a feature called projections, which allows transformed and pre-aggregated
data to exist on top of a base segment, and automatically be used by a query
against the base. Part of the needs of this is to match up virtual columns
present in the query to those present in the projections, so if a table exists
that already contains the transform we can use it and map projected virtual
column to the query virtual column to skip having to compute it. I don't
actually want to add this virtual column to `VirtualColumns`, though I should
probably precompute an equivalence set or something to use for better lookup.
The SQL layer currently handles deduplication of expressions before creating
VirtualColumns with `VirtualColumnRegistry`, so there isn't a super strong need
to have the same stuff here.
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