Dynamodb get items based on time and likes

Question

Currently, I am making a 9gag.com replica, where users can see posts having more than 20 votes from the last 2 months, how can I achieve this efficiently?

Here is my current design for post table

userID | createTime | totalUpvotes | imageUrl | title

where userID is the partition key and createTime is the sort key, I also need pagination so finally here is my parameter for fetching the items:

const params = {
    TableName: TABLE_NAME,
    FilterExpression:
      '#createTime >= :_createTime AND #totalUpvotes >= :_votes',
    ExpressionAttributeNames: {
      '#createTime': 'createTime',
      '#totalUpvotes': 'totalUpvotes'
    },
    ExpressionAttributeValues: {
      ':_createTime': Date.now() - FEED_TIME_THRESHOLD,
      ':_votes': votes
    },
    Limit: MAX_POST_FETCH
};
if (lastEvaluatedKey) params.ExclusiveStartKey = lastEvaluatedKey;
return db.scan(params).promise();

I know scan function is not recommended, but I am thinking of using Redis to cache the posts, and re-run the scan operation every 20 minutes or so since all users should have seen the same content on my website.

Answer 1

Create an attribute popular with a value that's present only for popular items (i.e. 20+ upvotes). You can use a simple constant as the value. Then create a (sparse) GSI having popular as the PK and createTime as the SK. You can then query the GSI for popular items with a createTime greater than two months ago.