Optimize database queries using the cursor object of SQLite

Use a background

I read a piece of code written by my colleagues very early , very interesting , In order to obtain the id aggregate , Through the first dao Object to get all personnel information in the database ,

Then one by one add Into the list aggregate . If the number of database reaches tens of thousands , In fact, whether in terms of memory or efficiency , This way is not advisable .

   /**
     *  Get personId aggregate 
     * todo  This method can be optimized 
     *
     * @return
     */
    public List<String> getPersonIdList() {
        List<PersonInfo> personInfos = queryAll();
        if (personInfos == null || personInfos.size() == 0) {
            return new ArrayList<>();
        }

        List<String> personIdsList = new ArrayList<>();
        for (PersonInfo info : personInfos) {
            if (info == null || TextUtils.isEmpty(info.getPersonId())) {
                continue;
            }
            personIdsList.add(info.getPersonId());
        }

        return personIdsList;
    }

Cursor object

Let's get to know Cursor object , He is right sqlite The result set after database query , It can be understood as a set of each row of query results .

Simple api Use as follows

// Close cursor , Release resources    
close()
        
// Retrieve the text of the requested column in the buffer , Store it 
copyStringToBuffer(int columnIndex, CharArrayBuffer buffer)
        
// Returns the total number of all columns 
getColumnCount()
        
// Returns the name of the specified column , Returns if none exists -1
getColumnIndex(String columnName)
        
// Returns the specified column name from zero , If it does not exist, it will throw IllegalArgumentException abnormal .
getColumnIndexOrThrow(String columnName)
        
// Returns the column name from the given index 
getColumnName(int columnIndex)
        
// Returns the column name of a string array 
getColumnNames()
        
// return Cursor The number of lines in 
getCount()
        
// Move the cursor to the first line 
moveToFirst()
        
// Move the cursor to the last line 
moveToLast()
        
// Move the cursor to the next line 
moveToNext()
     
// Move the cursor to an absolute position    
moveToPosition(int position)
        
 Move the cursor to the previous line 
moveToPrevious()
        

Use Cursor Optimize database queries

   /**
     *  Get personId aggregate 
     *
     * @return
     */
    public List<String> getPersonIdList() {
        List<String> personIdsList = new ArrayList<>();
        Cursor cursor = null;
        try {
            cursor = DbCore.getDaoSession().getRecordDao().getDatabase().rawQuery("select PERSON_ID from person_info", null);
            while (cursor.moveToNext()) {
                personIdsList.add(cursor.getString(0));
            }
            cursor.close();
        } catch (Exception e) {
            if (cursor != null) {
                cursor.close();
                cursor = null;
            }
        }

        return personIdsList;
    }

The above code can be seen through sql Statement to query only the required fields PERSON_ID Information , Then the data of each row passes cursor.getString(0) To get , In terms of memory allocation , This avoids building a large number of unnecessary personInfos object , In terms of reading efficiency , Also avoid querying unnecessary information .