Improve RowMetadata for presence/absence check of columns

[mp911de]

RowMetadata currently exposes getColumnMetadata(…) and getColumnMetadatas(). If a client wants to check if a column is part of the result, then it has to obtain getColumnMetadatas() and iterate over ColumnMetadata.

For a discovery-based client, that does not know the structure of a result, this mapping causes a significant overhead as the client cannot cache a previous metadata object. getColumnMetadata(…) on the other hand, throws an exception of the column at identifier (e.g. name of the column) is absent.

How about introducing a SortedSet<String> getColumnNames() as short-cut? This would solve at least three issues:

1. Discovery of the number of columns in the row.
2. Fast retrieval of column names without additional mapping.
3. Clients can easily call getColumnNames().contains(…) to check whether a column exists.

A nice side effect is that drivers can implement naming rules themselves (e.g. via TreeSet and a Collation) and drivers can cache the result metadata based on vendor-specific rules.

See also spring-projects/spring-data-r2dbc#69 for a use-case.

[nbenjamin]

@mp911de created a PR for this change #58. Please review and let me know if any changes are required

[nebhale]

A nice side effect is that drivers can implement naming rules themselves (e.g. via TreeSet and a Collation)

@mp911de Can you please elaborate on this a little?

[mp911de]

Databases typically implement their own rules how they compare field names (case-sensitive, case-insensitive) and they allow modifiers to (like escape chars) to enforce the former or the latter comparison method. By using an appropriate Comparator, drivers can implement column name equality rules, might resemble functionality of what Spring's LinkedCaseInsensitiveMap does.

[nebhale]

Understood. I'm good with this proposal then and am OK merging #58.