findepi commented on code in PR #6582:
URL: https://github.com/apache/iceberg/pull/6582#discussion_r1072412666
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core/src/main/java/org/apache/iceberg/puffin/StandardBlobTypes.java:
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@@ -26,4 +26,6 @@ private StandardBlobTypes() {}
* href="https://datasketches.apache.org/";>Apache DataSketches</a> library
*/
public static final String APACHE_DATASKETCHES_THETA_V1 =
"apache-datasketches-theta-v1";
+
+ public static final String NDV_BLOB = "ndv-blob";
Review Comment:
> Spark doesn't use Apache DataSketches to collect approximate NDV
Same was true for Trino. Trino uses HLL by default.
I introduced DataSketches Theta aggregation so that we can be compatible.
For that I had to revamped stats collection SPI so that connectors can request
desired sketch format, as previously it was hard-coded: if a connector wants
NDV information they get a HLL sketch.
For NDV information, without update'able sketch, we shouldn't use a blob at
all. The NDV number is just a property of actual updateabke sketch stored in
the Puffin file. For a POC, you can use a fake blob with empty content, and
associated NDV number as its property. Just give it some blob type name that
reveals it's a fake temporary. For production use, we want to write Theta
sketches.
cc @rdblue
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