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docs: clarify explain and fallback configuration options #2983

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24 changes: 24 additions & 0 deletions README.md
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Expand Up @@ -84,6 +84,30 @@ Comet aims for 100% compatibility with all supported versions of Apache Spark, a
your existing Spark deployments and workflows seamlessly. With no code changes required, you can immediately harness
the benefits of Comet's acceleration capabilities without disrupting your Spark applications.

## Explain and Fallback Configuration

Comet provides several configuration options to help users understand which parts of a Spark query plan are accelerated natively and when execution falls back to Spark.
These options control how fallback information and explain output are reported, and their behavior may vary depending on the Spark version.

The following configuration properties are commonly used when inspecting fallback behavior and explain output:

| Configuration | What it does | Notes |
|-------------------------------------------|--------------------------------------------------------|--------------------------------------------------------|
| `spark.comet.explainFallback.enabled` | Reports fallback reasons and shows an extended explain | No output is produced if there are no fallback reasons |
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@wForget wForget Dec 25, 2025

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These configurations are already in the comet configuration documentation, do we need to make this change?

By the way, the comet configuration documentation is automatically generated by CometConf.allConfs.

plan when fallback occurs
| `spark.comet.logFallbackReasons.enabled` | Logs fallback reasons to a WARN log entry | Does not include the associated query plan |
| `spark.comet.explain.verbose.enabled` | Enables more detailed explain output | Used with `ExtendedExplainInfo` (Spark 4.0+) |
| `spark.comet.explain.native.enabled` | Shows explain output for native execution | Output is written to executor logs and can be verbose |

### Spark Version Notes

Prior to Spark 4.0, verbose explain output is only shown when fallback occurs. Starting with Spark 4.0, Spark introduces support for `ExtendedExplainInfo`,
which allows verbose explain output even when no fallback occurs,
provided the appropriate configuration is enabled (for example, by setting `spark.sql.extendedExplainProviders`).

Because these configuration options affect logging and explain output in different ways, users are encouraged
to choose the settings that best match their debugging and observability needs.

## Tight Integration with Apache DataFusion

Comet tightly integrates with the core Apache DataFusion project, leveraging its powerful execution engine. With
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