Sorting in tarantool with min() if several records have equal secondary index

Question

  local orders = box.schema.space.create('orders')
  box.schema.sequence.create('orderId')
  orders:create_index('id', {sequence='orderId'})
  orders:create_index('price', {unique=false, parts={2, 'integer'}})

  local bestOrder = orders.index.price:min()

I'm searching best order (minimal price) with min() function over secondary index. How is Tarantool sorting records if they have the same price (secondary index)? I tested and look like by primary index. Is this behaviour standardized?

score 3 · Accepted Answer · edited Mar 22 '18 at 08:20

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Tarantool has several types of indexes [1]. And some of are sorted some of are not sorted. For instance, the tree index [2] will be sorted, that means you can get bounds, and the hash index [3] is not sorted, means you can get bounds without copy and sort.

In your case, you use a tree index. So you can use min, max, bound selects etc for 'price' index.

UPDATE (after had some talks via email)

Tarantool returns first tuple by primary key for min() if several records have equal secondary index and last for max().

This behavior is normal for Tarantool, and you can count that it would not be changed in the near future.

[1] https://tarantool.org/en/doc/2.0/book/box/box_index.html?highlight=index#module-box.index

[2] https://en.wikipedia.org/wiki/B-tree

[3] https://en.wikipedia.org/wiki/Hash_table

edited Mar 22 '18 at 08:20

farwayer

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answered Mar 13 '18 at 15:37

Vasiliy Soshnikov

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Default index type is tree by this docs https://tarantool.org/en/doc/2.0/book/box/box_space.html#box-space-create-index And `min()` plays well. – farwayer Mar 13 '18 at 15:49
Thanks, mistyped. Fixed. it is so – Vasiliy Soshnikov Mar 13 '18 at 15:55
But I don't know how tarantool sorts tuples with equal second indexes (price is 1 for several records). By primary index? – farwayer Mar 13 '18 at 16:01
In your case, tarantool uses the secondary index ('price'), which is b-tree. But if you try to select SPACE:select({}), then sorted order will be by primary key. – Vasiliy Soshnikov Mar 13 '18 at 23:28

Sorting in tarantool with min() if several records have equal secondary index

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