[FLINK-24889][table] Flink SQL Client should print correсtly multisets

[snuyanzin]

What is the purpose of the change

The PR allows to pretty print for multisets

This change added tests and can be verified as follows:

A case for multiset has been added to org.apache.flink.table.utils.PrintUtilsTest#testNestedRowToString

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• The S3 file system connector: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

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CI report:

• 9fb3806 Azure: SUCCESS

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[slinkydeveloper]

Hi, thank you for your pr.
PrintUtils doesn't exist anymore as part of the refactoring here https://issues.apache.org/jira/browse/FLINK-21456. In order to support MULTISET to STRING, now you need to add a rule like that https://github.com/apache/flink/blob/da24fbf23c244771084bf0a31cdbb9749d72e80a/flink-table/flink-table-planner/src/main/java/org/apache/flink/table/planner/functions/casting/ArrayToStringCastRule.java

[snuyanzin]

Hi @slinkydeveloper thanks for letting know about mentioned refactoring.
I've rebased branch to latest master and added a rule for multiset based on map as multiset is a map<t, Integer> as also mentioned in org.apache.flink.table.types.logical.MultisetType

[slinkydeveloper]

Given the casting logic is exactly the same, I don't think we need to have a separate rule for Multiset and Map. Just rename the map rule to MapAndMultisetToStringCastRule, change the predicate and make sure the valueType is the correct one.

[snuyanzin]

@slinkydeveloper thanks for your feedback.
I've made changes you've requested, could you please have a look once again.

P.S. I have no idea why Re-request review button does not work for me...

[slinkydeveloper]

Left a last minor nit comment, and then once the CI is green we can merge it. Thank you for the contribution!

[snuyanzin]

thanks for the review

[twalthr]

Sorry for jumping in so late. The PR looks good. I was just wondering if we would like to skip the null checking logic for values. Multiset values have always an integer. We can make the implementation a bit more efficient by skipping line 204 in MapAndMultisetToStringCastRule.

[slinkydeveloper]

@twalthr Perhaps the point here is that valueIsNullTerm should always be false if valueType is not null, because null checks shouldn't be required?

[twalthr]

Thanks for the update @snuyanzin. LGTM