AssetFromDB produces an xdr.Asset by combining the constituent type, code and issuer, as often retrieved from the DB in 3 separate columns.

Account is a row of data from the `accounts` table.

AccountData is a row of data from the `accountdata` table.

LedgerHeader is row of data from the `ledgerheaders` table.

Offer is row of data from the `offers` table from stellar-core.

OrderBookSummaryPriceLevel is a collapsed view of multiple offers at the same price that contains the summed amount from all the member offers.

PriceLevel represents an aggregation of offers to trade at a certain price.

Q is a helper struct on which to hang common queries against a stellar core database.

SequenceProvider implements `txsub.SequenceProvider`.

Signer is a row of data from the `signers` table from stellar-core.

Transaction is row of data from the `txhistory` table from stellar-core.

TransactionFee is row of data from the `txfeehistory` table from stellar-core.

Trustline is a row of data from the `trustlines` table from stellar-core.

OrderBookSummary is a summary of a set of offers for a given base and counter currency.