Data Management Development

How do I encrypt an RDB store? (API version 9)

Solution

To encrypt an RDB store, set encrypt in StoreConfig to true when creating the RDB store.

Reference

RDB Store

What should I do if the table data in an RDB store cannot be cleared by using TRUNCATE TABLE? (API version 9)

Symptom

An error is reported when the TRUNCATE TABLE statement is used to clear table data.

Solution

The RDB store uses SQLite, which does not support the TRUNCATE TABLE statement. To clear a table in an RDB store, use the DELETE statement, for example, DELETE FROM sqlite_sequence WHERE name = ‘table_name’.

What data types are supported by an RDB store? (API version 9)

Applicable to: stage model

Solution

An RDB store supports data of the number, string, and Boolean types. The number type supports data of the Double, Long, Float, Int, or Int64 type, with a maximum precision of 17 decimal digits.

How do I persistent application data? (API version 9)

Solution

You can use the PersistentStorage class to implement application data persistence. You can link the persistent data with specific tags to AppStorage, and invoke AppStorage APIs to access the persistent data. Persistent data is stored in a local XML file in /data/app/el2/100/base/<bundleName>/haps/<hapName>/files/persistent_storage.

Example:

AppStorage.Link('varA')
PersistentStorage.PersistProp("varA", "111");
@Entry
@Component
struct Index {
  @StorageLink('varA') varA: string = ''
  build() {
    Column() {
      Text('varA: ' + this.varA).fontSize(20)
      Button('Set').width(100).height(100).onClick(() => {
        this.varA += '333'
      })
    }
    .width('100%')
    .height('100%')
  }
}

Reference

PersistentStorage: Application State Persistence

How do I save pixel map data to a database? (API version 9)

Solution

Convert the pixel map data into an ArrayBuffer and save the ArrayBuffer to your database.

Reference

readPixelsToBuffer

How do I obtain RDB store files? (API version 9)

Symptom

Problem of obtaining RDB store files.

Solution

The RDB store files are stored in /data/app/el2/100/database/Bundle_name/entry/rdb/. You can use the hdc command to copy the file from the directory and use a SQLite tool to open the file.

Example:

 hdc file recv /data/app/el2/100/database/<bundleName>/entry/db/<tableName>  ./<path>

Should I design a lock mechanism for databases in development? (API version 9)

Symptom

I do not know whether I need to design a lock mechanism for databases in development.

Solution

The distributed data service (DDS), RDB store, and preferences provided by OpenHarmony have a lock mechanism. You do not need to bother with the lock mechanism during the development.

What should I do if I failed to use get() to obtain the data saved by @ohos.data.storage put()? (API version 9)

Symptom

After @ohos.data.storage put() is called to save data, get() is called to obtain the data. However, the data fails to be obtained.

Solution

The put() method provided by @ohos.data.storage saves data in the memory. When the application exits, the data in the memory will be cleared. If you want to persist the data, you need to call flush() or flushSync() after put(). After data is persisted, you can use get() to obtain the data after the application is restarted.

What should I do if a large text file fails to be saved in an RDB store? (API version 9)

Symptom

In API version 8, large text files cannot be saved in RDB stores.

Solution

In versions earlier than API version 9, the maximum length of a text file is 1024 bytes. If the text file exceeds 1024 bytes, it cannot be saved.

The limit on the text file size has been removed since API9 version.

What should I do if “undefined” is returned by Preferences.get after Preferences.put is successful? (API version 9)

Symptom

Data is successfully saved using Preferences.put(), but fails to be obtained by Preferences.get().

Solution

1. After put() is performed, use flush() to persist the data.

2. Call get().

Can I specify the in-memory database mode when using an RDB store? (API version 9)

Solution

RDB stores use SQLite. The default in-memory database mode is file, which cannot be changed.

How do I obtain the size of an RDB store?

Solution

You can use execute to obtain the size of an RDB store, for example, SELECT page_count * page_size AS size FROM pragma_page_count(), pragma_page_size().

How do I obtain the path of an RDB store?

Solution

1. If you has the root permission, run the find /data -name database_name command.

2. You can use File Browser of DevEco Studio to obtain the database file path.

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