[VL] Check whether the write is a supported iceberg write before executing offload

[Zouxxyy]

What changes are proposed in this pull request?

Check whether the write is a supported iceberg write before executing

How was this patch tested?

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[Zouxxyy]

I found that with the current offload + validate fallback:
Suppose we have two components offloading the AppendExec, for example, OffloadIcebergWrite and OffloadPaimonWrite. If validation fails for VeloxIcebergAppendDataExec, AppendDataExec will no longer be passed to OffloadPaimonWrite. Is this by designed, or am I missing something? @zhztheplayer thanks

[Zouxxyy]

I added reflection here to ensure that this rule can still execute correctly when there's no Iceberg dependency. A similar approach is used in offloadBatchScan.

[zhztheplayer]

Is this by designed, or am I missing something? @zhztheplayer thanks

Yes, this sounds like one of the major issues we have for making different components (e.g., paimon + iceberg) work together. This validator was written for keeping up with some legacy code in our query planner, and I do think it should be eventually removed or redesigned.

We should make one component try to offload a plan node that the previous component couldn't handle, not skip it.

Maybe we can just make an attempt to remove that validator to see what happens then.

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[Zouxxyy]

We should make one component try to offload a plan node that the previous component couldn't handle, not skip it.

I see, this sounds great!

This validator was written for keeping up with some legacy code in our query planner, and I do think it should be eventually removed or redesigned.

Do we have a plan for this part.

[zhztheplayer]

Do we have a plan for this part.

I haven't had a clear plan on it, perhaps I can take a closer look next week or so. Please feel free to take it on if you are interested.

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