As far as I can tell they're not storing anything locally, they're pulling the data from Google Cloud Storage as you access it.
That said, a Wasm module doesn't have access to any storage facilities that the browser doesn't already expose (IndexedDB, OPFS, cookies, etc.) so even if it could be used by marketers, they would gain nothing by using DuckDB for that over just using the same underlying browser storage API.
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As far as I can tell they're not storing anything locally, they're pulling the data from Google Cloud Storage as you access it.
That said, a Wasm module doesn't have access to any storage facilities that the browser doesn't already expose (IndexedDB, OPFS, cookies, etc.) so even if it could be used by marketers, they would gain nothing by using DuckDB for that over just using the same underlying browser storage API.