CODEWITHSUNDEEP
mongodbshardingunique-indexcompound-index
intellectual
intellectual

Reputation: 1

What is the mongo compound index keyword arrangement rule? Numbers first, strings later?

When I used numbers as keys, the compound index prefixes created were not as expected。

To define a compound index:

admin> use TestDB
TestDB> db.createCollection('coll')
TestDB> db.coll.createIndex({4:1,1:-1},{unique:true})
TestDB> db.coll.getIndexex()

Output

[
  { v: 2, key: { _id: 1 }, name: '_id_' },
  { v: 2, key: { '1': -1, '4': 1 }, name: '1_-1_4_1', unique: true }
]

I expected 4 to be the prefix of the index, but it turned out to be 1, why? It looks like the sorting happens

And when I use strings as keywords, the results are completely inconsistent。

Following：

TestDB> db.createCollection("coll2")
TestDB> db.coll2.createIndex({'s':1, 'a':-1},{unique:true})

Output

[
  { v: 2, key: { _id: 1 }, name: '_id_' },
  { v: 2, key: { s: 1, a: -1 }, name: 's_1_a_-1', unique: true }
]

What? It doesn't seem to be sorting.

Upvotes: 0

Views: 32

Answers (1)

user20042973
user20042973

Reputation: 5065

According to this issue, which references the official specification, this is a result of how Javascript works.

For example, in Node:

$ node
Welcome to Node.js v16.17.0.
Type ".help" for more information.
> var pattern = { 4: 1, 1: -1 }
undefined
> pattern
{ '1': -1, '4': 1 }

You can see similar behavior in this jsfiddle.

If you are attempting to create indexes that have numeric keys, you will need to do so with a different language/driver.

Upvotes: 1

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