Query in DynamoDB using Java API

Question

I need to implement a Query operation on DynamoDB. Right now I'm doing it by giving the HashKey and then filtering out the results according to my conditions on non-key attributes.

This is what I'm doing :

MusicData hashKey = new MusicData();
hashKey.setID(singer); 

DynamoDBQueryExpression<MusicData> queryExpression = new DynamoDBQueryExpression<MusicData>().withHashKeyValues(hashKey);

List<MusicData> queryResult = mapper.query(MusicData.class, queryExpression);

for (MusicData musicData : queryResult) {
    if( my condtions ) {
        do something;   
    }
}

What I'm trying to do is to be able to do something like this :

MusicData hashKey = new MusicData();
hashKey.setID(singer); 
hashKey.setAlbum(sampleAlbum);
hashKey.setSinger(duration);

DynamoDBQueryExpression<MusicData> queryExpression = new DynamoDBQueryExpression<MusicData>().withHashKeyValues(hashKey);

List<MusicData> queryResult = mapper.query(MusicData.class, queryExpression);

for (MusicData musicData : queryResult) {
    if( my condtions ) {
        do something;   
    }
}

And get results already filtered out. Is there a way to do this in DynamoDB?

JaredHatfield · Accepted Answer · 2015-07-28 00:08:40Z

1

Yes, you can ask DynamoDB to perform filtering of queries before it returns results. However, you will still incur the 'cost' of reading these items even though they are not returned to your client. This is still a good practice as it will eliminate unnecessary transfer of items over the network.

To do this you will call additional methods on your DynamoDBQueryExpression object, specifically withFilterExpression and addExpressionAttributeNamesEntry / addExpressionAttributeValuesEntry to complete the expression.

Without the specific example of what type of conditions you want to apply it is hard to give an example, but depending on how simple your condition you want to apply is you could contain it in just the withFilterExpression method.

DynamoDBQueryExpression<MusicData> queryExpression = new DynamoDBQueryExpression<MusicData>().withHashKeyValues(hashKey).withFilterExpression("foo > 10");

answered Jul 28, 2015 at 0:08

JaredHatfield

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2 Comments

dushyantashu Over a year ago

Okay, I was looking for a way to extract only those items according to my condition, rather than getting all Items and then filtering out on my conditions.

JaredHatfield Over a year ago

If your condition is only on the range key that is possible using a different method, but without the specifics what I described above is exactly what you are asking for.

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