ri_RootToPartitionMap is currently only initialized for tuple routing
target partitions, though a future commit will need the ability to use
it even for the non-partition child tables, so make adjustments to the
decouple it from the partitioning code.
Also, make it lazily initialized via ExecGetRootToChildMap(), making
that function its preferred access path. Existing third-party code
accessing it directly should no longer do so; consequently, it's been
renamed to ri_RootToChildMap, which also makes it consistent with
ri_ChildToRootMap.
ExecGetRootToChildMap() houses the logic of setting the map appropriately
depending on whether a given child relation is partition or not.
To support this, also add a separate entry point for TupleConversionMap
creation that receives an AttrMap. No new code here, just split an
existing function in two.
Author: Amit Langote <amitlangote09@gmail.com>
Discussion: https://postgr.es/m/CA+HiwqEYUhDXSK5BTvG_xk=eaAEJCD4GS3C6uH7ybBvv+Z_Tmg@mail.gmail.com
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