Redisearch aggregate return top 5 of each group

Question

Suppose I have documents in this format:

product_name TEXT tags TAG score NUMERIC 

[product1, [tag1, tag2, tag3], 10]
[product2, [tag2, tag3, tag4], 100]
....
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Answer 1

First:

• Make sure to disable features you won't use (NOOFFSETS, NOHL, NOFREQS, STOPWORDS 0)
• Use SORTABLE for your NUMERIC score.

Here is the schema I used to test:

FT.CREATE product_tags NOOFFSETS NOHL NOFREQS STOPWORDS 0
    SCHEMA product_name TEXT tags TAG score NUMERIC SORTABLE

You want to think of FT.AGGREGATE as a pipeline.

The first step will be to sort the products by @score, so that later, down in the pipeline, when we REDUCE TOLIST 1 @product_name, the list comes out sorted:

SORTBY 2 @score DESC

I think you are already doing LOAD/APPLY to deal with the tags, as TAG fields would otherwise be grouped by the full comma-separated string tags-list, per product. See Allow GROUPBY on tag fields issue. So our next step is in the pipeline is:

LOAD 1 @tags 
APPLY split(@tags) as TAG 

We then group by @TAG, and apply the two reductions. Our products list will come out sorted.

GROUPBY 1 @TAG
    REDUCE SUM 1 @score AS total_score
    REDUCE TOLIST 1 @product_name AS products

Finally, we sort by @total_score:

SORTBY 2 @total_score DESC

Here a final view of the command:

FT.AGGREGATE product_tags *
    SORTBY 2 @score DESC 
    LOAD 1 @tags 
    APPLY split(@tags) as TAG
    GROUPBY 1 @TAG
        REDUCE SUM 1 @score AS total_score 
        REDUCE TOLIST 1 @product_name AS products
    SORTBY 2 @total_score DESC

Here a full list of commands to illustrate the result. I used productXX with score XX to easily verify visually the sorting of products.

> FT.CREATE product_tags NOOFFSETS NOHL NOFREQS STOPWORDS 0 SCHEMA product_name TEXT tags TAG score NUMERIC SORTABLE
OK
> FT.ADD product_tags pt:product10 1 FIELDS product_name product10 tags tag2,tag3,tag4 score 10
OK
> FT.ADD product_tags pt:product1 1 FIELDS product_name product1  tags tag1,tag2,tag3 score 1
OK
> FT.ADD product_tags pt:product100 1 FIELDS product_name product100 tags tag2,tag3 score 100
OK
> FT.ADD product_tags pt:product5 1 FIELDS product_name product5 tags tag1,tag4 score 5
OK
> FT.SEARCH product_tags *
1) (integer) 4
2) "pt:product5"
3) 1) "product_name"
   2) "product5"
   3) "tags"
   4) "tag1,tag4"
   5) "score"
   6) "5"
4) "pt:product100"
5) 1) "product_name"
   2) "product100"
   3) "tags"
   4) "tag2,tag3"
   5) "score"
   6) "100"
6) "pt:product1"
7) 1) "product_name"
   2) "product1"
   3) "tags"
   4) "tag1,tag2,tag3"
   5) "score"
   6) "1"
8) "pt:product10"
9) 1) "product_name"
   2) "product10"
   3) "tags"
   4) "tag2,tag3,tag4"
   5) "score"
   6) "10"
> FT.AGGREGATE product_tags * SORTBY 2 @score DESC LOAD 1 @tags APPLY split(@tags) as TAG GROUPBY 1 @TAG REDUCE SUM 1 @score AS total_score REDUCE TOLIST 1 @product_name AS products SORTBY 2 @total_score DESC
1) (integer) 4
2) 1) "TAG"
   2) "tag2"
   3) "total_score"
   4) "111"
   5) "products"
   6) 1) "product100"
      2) "product10"
      3) "product1"
3) 1) "TAG"
   2) "tag3"
   3) "total_score"
   4) "111"
   5) "products"
   6) 1) "product100"
      2) "product10"
      3) "product1"
4) 1) "TAG"
   2) "tag4"
   3) "total_score"
   4) "15"
   5) "products"
   6) 1) "product10"
      2) "product5"
5) 1) "TAG"
   2) "tag1"
   3) "total_score"
   4) "6"
   5) "products"
   6) 1) "product5"
      2) "product1"

You are getting the full list of products sorted, not just the top 5. Complexity-wise it makes no difference, we paid the price. The impact is in buffering, network payload, and your client.

You can limit to top 5 using a Lua script:

eval "local arr = redis.call('FT.AGGREGATE', KEYS[1], '*', 'SORTBY', '2', '@score', 'DESC', 'LOAD', '1', '@tags', 'APPLY', 'split(@tags)', 'as', 'TAG', 'GROUPBY', '1', '@TAG', 'REDUCE', 'SUM', '1', '@score', 'AS', 'total_score', 'REDUCE', 'TOLIST', '1', '@product_name', 'AS', 'products', 'SORTBY', '2', '@total_score', 'DESC') \n for i=2,(arr[1]+1) do \n arr[i][6] = {unpack(arr[i][6], 1, ARGV[1])} \n end \n return arr" 1 product_tags 5

Here a friendly view of the Lua script above:

local arr = redis.call('FT.AGGREGATE', KEYS[1], ..., 'DESC')
for i=2,(arr[1]+1) do 
    arr[i][6] = {unpack(arr[i][6], 1, ARGV[1])}
end
return arr

We are passing one key (the index) and one argument (the limit for top products, 5 in your case): 1 product_tags 3.

With this, we limited the impact to buffering only, saved network payload and load on your client.