Tarantool: limit/offset in index.indexName:pairs call

Question

I need to fetch some records from space users. This space has a secondary index category_status_rating. I need select users with category=1, status=1, rating<=123456789:

for _, user in box.space.users.index.category_status_rating:pairs({ 1, 1, 123456789 }, { limit = 20, offset = 5, iterator = box.index.LE }) do
    if user[categoryIdx] ~= 1 or user[statusIdx] ~= 1 then break end
    table.insert(users, user)
end

As I know, iteration with indexName:pairs does not support limit and I can just user my own counter. But what about offset? Can I use this param and start from "page" I need? Or will I iterate without any offset and pass useless records (about 100000) and start to table.insert(users, user) when my "page" starts? Thanks!

why don't you just try if it does what you want? – Piglet Mar 30 '16 at 12:06 — Piglet, Mar 30 '16 at 12:06

score 3 · Accepted Answer · answered Mar 30 '16 at 13:40

Instead of using offset, you can save your position (that will be last checked tuple) if you really need it. e.g:

local last = 123456789
for i = 1, 2 do
    local count = 0
    for _, user in box.space.users.index.category_status_rating:pairs({1, 1, last}, { iterator = box.index.LE }) do
        if user[categoryIdx] ~= 1 or user[statusIdx] ~= 1 or count > 20 then
            break
        end
        table.insert(users, user)
        last = user[LAST_INDEX_FIELD]
        count = count + 1
    end
    -- process your tuples
end

or, using luafun (where drop_n is analog of limit, and saving into last is analog of offset):

local last = 123456789
for i = 1, 2 do
    local users = box.space.users.index.category_status_rating:pairs({1, 1, last}, { iterator = box.index.LE }):take_n(20):map(function(user)
        last = user[LAST_INDEX_FIELD]
        return user
    end):totable()
    -- process your tuples
end

Documentation on LuaFun, which is embedded into Tarantool.

Hmm... But what if `rating` is not an unique value? Does it mean that records from `first 20 records` can be in `second 20 records` if I willll use `last` as position? — Levintsev Aleksandr, Mar 30 '16 at 16:24
Then you'll need to count number of tuples with rating `last` that you processed, and (for example) use `drop_n` — bigbes, Mar 31 '16 at 07:54

Tarantool: limit/offset in index.indexName:pairs call

1 Answers1