Derive Uniswap.exchange rather than Exchange.factory

[davekay100]

See the condensed schema below for Uniswap and Exchange

# Uniswap global values across all exchanges
type Uniswap @entity {
    id: ID!
    exchanges: [Exchange!]!
    .........
}

type Exchange @entity {
    id: ID!                                 # Uniswap Exchange address
   ...........
    # Fields used to help derived relationship
    factory: Uniswap! @derivedFrom(field: "exchanges")
}

This is a bad pattern because we are deriving factory from exchange, while factory -> exchange has a relationship of 1-n . Right now we are deriving the factory from the exchanges, even though we know factory will always have id 1 and that there is only one factory. We should do the following:

# Uniswap global values across all exchanges
type Uniswap @entity {
    id: ID!
    exchanges: [Exchange!]! @derivedFrom(field: "factory")
    ......
}

type Exchange @entity {
    id: ID!                                 # Uniswap Exchange address
   ...........
    factory: Uniswap! 
}

And set each Exchanges factory value to the string 1. This will allow for simpler sql queries within graph-node

[lutter]

Changing this is particularly pressing for Transaction and how events link to it as the current setup is causing a lot of very slow queries right now, and I haven't figured out a way to speed them up with additional indexes etc. If, for example, AddLiquidityEvent stored the Transaction it belongs to, and Transaction.addLiquidityEvents was derived, those queries would run a lot faster.