optimizer: merge StreamIndexScan into StreamTableScan

[BugenZhao]

StreamIndexScan was introduced for delta-join in #2107. But as we refactor it, it seems there's not much difference in the implementation between IndexScan and TableScan, especially after we introduce the ChainType. Should we merge them into one to avoid code duplication?

cc @chenzl25 @st1page @fuyufjh

[fuyufjh]

Sounds reasonable. 🤔 What's the initial motivation of introducing StreamIndexScan?

[chenzl25]

Yes, I think they have no essential difference.

[chenzl25]

Sounds reasonable. 🤔 What's the initial motivation of introducing StreamIndexScan?

IMO, creating a delta join on existing mv just needs to consume existing data from one data flow. FYI, N way delta join has N
separate data flows. Previously, it is likely that the index scan use a different strategy to consume the existing data.

[y-wei]

close as completed #8567