[SPARK][VARIANT] Preserve Variant stats from JSON addFiles during checkpointing #5916
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Which Delta project/connector is this regarding?
Description
This PR adds support for preserving variant stats from JSON AddFile records during checkpointing. Since stats are stored in Z85 encoding, they need to be decoded after the
from_jsonexpression. This PR implements theDecodeNestedZ85EncodedVariantexpression for decoding variant stats from the Z85 representation.We need to implement custom logic for stats stored as structs in previous checkpoints. However, currently, we don't preserve stats_struct for any data type, let alone variant. This PR fixes this for other data types. Once that PR goes in, I will make another PR to preserve stats from previous checkpoints.
TODO: Hide behind shims for Spark 4.0 compatibility
How was this patch tested?
Using golden files. We copied golden tables into this project where the AddFiles contain variant stats and tested that checkpointing on this table preserves variant stats.
Does this PR introduce any user-facing changes?
Yes