What is a block index in blockchain.info? On blockchain's blockchain api documentation, located here

https://blockchain.info/api/blockchain_api


There is an api call that uses a parameter called block_index. What is this value, and how does it differ from block_height? From playing around with changing the block_index value in the following api call:

https://blockchain.info/block-index/$block_index?format=json


It seems like (imo quite arbitrarily, which adds to my confusion) that block_index 14849 corresponds to the genesis block. Finally, on the api page listed at the top the first example output they show gives a block whose index is 818044, yet when I enter the following api call into my browser's address bar:

https://blockchain.info/block-index/818044?format=json


I get "Block Not Found". Is this just a bug in blockchain.info that the example output they provide doesn't actually work, or am I using block_index incorrectly?
I think that this value is an internal unique index in bc.i database.

It is not possible to use txid as an index because of 2x2 duplicate transactions in blockchain

https://blockchain.info/tx/d5d27987d2a3dfc724e359870c6644b40e497bdc0589a033220fe15429d88599
https://blockchain.info/tx/e3bf3d07d4b0375638d5f1db5255fe07ba2c4cb067cd81b84ee974b6585fb468

occurred in the early stage of bitcoin

see BIP-30 https://github.com/bitcoin/bips/blob/master/bip-0030.mediawiki

UPD: ok, you are asking about block index, but my answer is for tx index. May be using straight autoincrement int64 value for database is better than int256