Description

Currently in Velox, we pick the first split where its stete is KPrepared, not the one with most data fetched. Once it's picked, we will need to wait until its data return later, even the other splits already have ready data.

Ideally we should wait there until the first split has its first rowgroup data returned, but the logic is too complex. Instead we can pick the split which has most data returned in first rowgroup.

Gluten version

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