We decode the chain.
You build the index.

import { Index } from "@secondlayer/sdk";
import { on } from "@secondlayer/stacks/filters";
import { db } from "./index-db";

const index = new Index();

// One filter, written once — it drives the quick walk here
// and the production consumer in consumer.ts.
const sales = on.contractCall({
  contractId: "SPNWZ…E0VQ0S.marketplace-v4",
  functionName: "purchase-asset",
});

// Tail every decoded call — no node to run, no Clarity to parse.
for await (const call of index.contractCalls.walk(
  sales.toContractCallsParams(),
)) {
  const [collection, tokenId] = call.args;

  await db
    .insertInto("sales")
    .values({
      tx_id: call.tx_id,
      buyer: call.sender,
      collection: String(collection),
      token_id: String(tokenId),
      block_height: call.block_height,
    })
    .execute();
}