When working with Room, a powerful persistence library in Android, you might encounter warnings about unused columns in your query results. These warnings can be a bit daunting, but they are actually quite helpful in optimizing your app’s performance. In this article, we will explore how to handle these warnings and ensure your Room queries are as efficient as possible.

The Problem

Consider the following warning message:

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/Users/guanyc/StudioProjects/BornWinClean/app/build/tmp/kapt3/stubs/debug/com/guanyc/stock/discipline/data/local/dao/StockNoteDao.java:118:
warning: The query returns some columns
[color, isCompleted, hasUnplannedAction, reviewMarket, string1, string2, string3, string4, string5, b1, b2, b3, b4, b5, i1, i2, i3, i4, i5, f1, f2, f3, f4, f5]
which are not used by com.guanyc.stock.discipline.presentation.main.MainViewModel.StockNotePanelPojo.
You can use @ColumnInfo annotation on the fields to specify the mapping.
You can annotate the method with @RewriteQueriesToDropUnusedColumns
to direct Room to rewrite your query to avoid fetching unused columns.

You can suppress this warning by annotating the method with @SuppressWarnings(RoomWarnings.CURSOR_MISMATCH).
Columns returned by the query: stockNoteId, createDate, color, isCompleted, hasUnplannedAction,
reviewMarket, string1, string2, string3, string4, string5,
b1, b2, b3, b4, b5, i1, i2, i3, i4, i5, f1, f2, f3, f4, f5.

This warning from Room indicates that your query is retrieving columns that are not actually used in the StockNotePanelPojo class. Room offers a few solutions for handling this:

Solutions

  1. Map Only Used Columns with @ColumnInfo Annotations You can add @ColumnInfo annotations to each field in StockNotePanelPojo to specify exactly which columns are needed from the database. This approach keeps your query efficient by making it clear which columns to retrieve. For example:

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    public class StockNotePanelPojo {
    @ColumnInfo(name = "stockNoteId")
    public int stockNoteId;

    @ColumnInfo(name = "createDate")
    public Date createDate;

    // Map only the necessary columns here
}

  2. Direct Room to Drop Unused Columns with @RewriteQueriesToDropUnusedColumns You can add this annotation to the query method in your DAO. Room will then rewrite the query to exclude unused columns automatically:

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    @RewriteQueriesToDropUnusedColumns
@Query("SELECT * FROM StockNote WHERE ...")  // Your actual query here
List<StockNotePanelPojo> getStockNotes();

  3. Suppress the Warning if It’s Not Critical If the unused columns are unimportant and don’t impact performance, you could suppress the warning with @SuppressWarnings(RoomWarnings.CURSOR_MISMATCH). This approach is less recommended as it doesn’t resolve the underlying inefficiency.

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    @SuppressWarnings(RoomWarnings.CURSOR_MISMATCH)
@Query("SELECT * FROM StockNote WHERE ...")  // Your actual query here
List<StockNotePanelPojo> getStockNotes();

Choosing an Approach

  • If you plan to access the unused columns in the future, @RewriteQueriesToDropUnusedColumns is a good balance.
  • For a smaller, optimized query, using @ColumnInfo to specify exact mappings is usually best.

This will help eliminate the warning and make your query more efficient by only retrieving what you actually need in StockNotePanelPojo.

Conclusion

Handling unused columns in Room queries is a straightforward process that can significantly improve the performance of your Android app. By using the appropriate annotations and techniques, you can ensure that your queries are optimized and your app runs smoothly.

Happy coding!