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Introduces `PlaceholderExpr`, allowing placeholder parameters to be preserved in the physical plan. Previously, placeholders had to be resolved to literals before physical planning.
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Rationale for this change
Part of #14342.
Previously, DataFusion required all placeholders to be resolved to literal values before physical planning. This limitation made it difficult to work in systems where physical plans might be cached or reused, or where resolution happens later in the execution pipeline.
By introducing
PlaceholderExprat the physical level, we allow the physical planner to handle unresolved placeholders. This is a logical next step for PR #20009 and part of the effort to improve plan reuse support in DataFusion.What changes are included in this PR?
PlaceholderExpras a new physical expression.PlaceholderExprwhen encounteringExpr::Placeholder.Are these changes tested?
Yes, SLTs have been added, which verifies that placeholders are correctly preserved in physical plans across a wide range of SQL queries.
Are there any user-facing changes?
Yes, users can now generate physical plans for queries containing unresolved placeholders. Attempting to execute these plans without resolving the placeholders will still result in an error, but the planning phase no longer requires their resolution.