relationalStore.StoreConfig只要配置rootDir，无论什么可访问路径，都是只读模式。

• 如果只读，可以借助DocumentViewPicker，pickMode为DOWNLOAD获取Download目录权限打开db。
• 如果要读写，就得复制到databaseDir/rdb目录下，并且relationalStore.StoreConfig不能配置rootDir。

完整示例：

import { common } from '@kit.AbilityKit';
import { fileIo, fileUri, picker } from '@kit.CoreFileKit';
import { relationalStore } from '@kit.ArkData';
import { BusinessError } from '@kit.BasicServicesKit';
import { resourceManager } from '@kit.LocalizationKit';

@Entry
@Component
struct Index {
  private commonContext: common.UIAbilityContext = this.getUIContext().getHostContext() as common.UIAbilityContext;
  private promptAction = this.getUIContext().getPromptAction();
  private rdbStore: relationalStore.RdbStore | undefined = undefined;

  build() {
    Column({ space: 20 }) {
      Button('打开Download DB（只读）')
        .type(ButtonType.ROUNDED_RECTANGLE)
        .onClick(() => {
          this.getDownloadRdb(false,'test.db');
        });

      Button('复制打开Download DB（读写）')
        .type(ButtonType.ROUNDED_RECTANGLE)
        .onClick(() => {
          this.getDownloadRdb(true,'test.db');
        });

      Button('创建表')
        .type(ButtonType.ROUNDED_RECTANGLE)
        .onClick(() => {
          this.createTable();
        });
    }
    .justifyContent(FlexAlign.Center)
    .width('100%')
    .height('100%');
  }

  getDownloadRdb(write:boolean, dbName:string){
    let context = this.getUIContext().getHostContext() as common.UIAbilityContext;
    const documentViewPicker = new picker.DocumentViewPicker(context);
    documentViewPicker.save({ pickerMode:picker.DocumentPickerMode.DOWNLOAD }).then((documentSaveResult: string[]) => {
      const uri = new fileUri.FileUri(documentSaveResult[0]);
      if (write) {
        let srcFile = `${uri.path}/${dbName}`;
        let destFile = `${this.commonContext.databaseDir}/rdb/${dbName}`;//db放在这个文件夹
        let copied = this.copyFile(srcFile, destFile);
        if (copied) {
          let storeConfig: relationalStore.StoreConfig = {
            name: dbName,
            securityLevel: relationalStore.SecurityLevel.S1,
          };
          this.initDataBase(storeConfig);
        }
      }else{
        let storeConfig: relationalStore.StoreConfig = {
          name: dbName,
          securityLevel: relationalStore.SecurityLevel.S1,
          rootDir:uri.path,//配置此项, 只读
        };
        this.initDataBase(storeConfig);
      }
    })
  }

  copyFile(srcFilePath: string, targetFilePath: string): boolean {
    if (!fileIo.accessSync(srcFilePath)) {
      this.getUIContext().getPromptAction().showToast({ message: '源文件不存在' });
      return false;
    }
    let srcFile = fileIo.openSync(srcFilePath);
    let destFile = fileIo.openSync(targetFilePath, fileIo.OpenMode.READ_WRITE | fileIo.OpenMode.CREATE)
    fileIo.copyFile(srcFile.fd, destFile.fd).then(() => {
      this.getUIContext().getPromptAction().showToast({ message: '成功复制' });
    }).catch((error: BusinessError) => {
      this.getUIContext().getPromptAction().showToast({ message: '复制异常' + error.message });
      return false;
    }).finally(() => {
      fileIo.closeSync(srcFile.fd);
      fileIo.closeSync(destFile.fd);
    })
    return true;
  }

  initDataBase(config:relationalStore.StoreConfig) {
    try {
      relationalStore.getRdbStore(this.commonContext, config)
        .then(async (rdbStore: relationalStore.RdbStore) => {
          this.rdbStore = rdbStore;
          console.info('Get RdbStore successfully.');
          this.promptAction.showToast({ message: `数据库打开[或创建打开]成功` });
        })
        .catch((err: BusinessError) => {
          console.error(`Get RdbStore failed, code is ${err.code},message is ${err.message}`);
          this.promptAction.showToast({ message: `数据库操作失败` });
        });
    } catch (err) {
      this.promptAction.showToast({ message: `打开数据库出错` });
      console.error(`getRdb failed, code is ${err.code},message is ${err.message}`);
    }
  }

  async createTable(){
    if (!this.rdbStore) {
      this.promptAction.showToast({ message: `请先打开数据库` });
      return;
    }
    const SQL_CREATE_TABLE = 'CREATE TABLE IF NOT EXISTS PERSON (ID INTEGER PRIMARY KEY AUTOINCREMENT, NAME TEXT NOT NULL, AGE INTEGER)';
    try {
      await this.rdbStore?.executeSql(SQL_CREATE_TABLE);
      this.promptAction.showToast({ message: `成功建表` });
    } catch (error) {
      this.promptAction.showToast({ message: `建表失败` });
      console.error('建表失败')
    }
  }
}