[parquet] Add map shredding for hot keys

[Aitozi]

Purpose

Add Parquet map shredding support for MAP<STRING, T> columns.

This allows selected map columns to extract hot keys into independent physical Parquet columns while preserving the original logical map schema for readers. The feature is controlled by map.shredding.* options, aligned with the existing variant.shredding.* naming style. It also adds a focused round-trip test and a storage benchmark to validate the storage benefit.

Tests

• mvn -pl paimon-api,paimon-format -Pfast-build -DskipTests compile
• mvn -pl paimon-format -am -Pfast-build -DfailIfNoTests=false -Dtest=ParquetFormatReadWriteTest#testMapShreddingRoundTrip,MapShreddingStorageBenchmark test
• git diff --check

Physical Layout

This change does not introduce a new Parquet logical type and does not modify the standard Parquet MAP encoding. A shredded map is still written with the regular Parquet map group as the residual map. Hot keys are promoted into additional sibling sidecar columns in the parent Parquet group.

For example, a logical field:

headers MAP<STRING, STRING>

is normally written as:

message paimon_schema {
  optional group headers (MAP) {
    repeated group key_value {
      required binary key (STRING);
      optional binary value (STRING);
    }
  }
}

With map shredding enabled, if user-agent and host are selected as hot keys, the physical Parquet schema becomes:

message paimon_schema {
  optional group headers (MAP) {
    repeated group key_value {
      required binary key (STRING);
      optional binary value (STRING);
    }
  }

  optional binary dynamic_column_headers_value_0 (STRING);
  optional binary dynamic_column_headers_value_1 (STRING);
}

The footer metadata records the mapping from sidecar columns to map keys:

parquet.meta.dynamic.column.map.keys.of.headers = user-agent,host

During writing, entries for promoted hot keys are omitted from the residual map when their values are non-null, and their values are written into the corresponding sidecar columns. During reading, Paimon reads both the residual map and the sidecar columns, then reconstructs the original logical MAP<STRING, T> value.

For nested maps, the same rule applies within the containing row group. For example, for payload.headers, sidecar columns are added as siblings of the headers map inside the payload group, and the footer metadata uses the full logical path:

parquet.meta.dynamic.column.map.keys.of.payload.headers = user-agent,host

#7876

[Aitozi]

Benchmark command:

mvn -s ~/.m2/apache-community.xml -pl paimon-format -am -Pfast-build \
  -DfailIfNoTests=false -Dtest=MapShreddingStorageBenchmark test

Benchmark file: [MapShreddingStorageBenchmark.java]

Common Setup

• Schema: id INT, headers MAP<STRING, STRING>
• Rows: 100,000
• Hot keys: 32
• Value length: 16
• Compression: snappy
• Compared layouts:
  • regular: normal Parquet map encoding
  • mapShredding: promotes 32 hot keys from headers into sidecar columns
• Map shredding options:
  • map.shredding.columns=headers
  • map.shredding.maxKeys=32
  • map.shredding.maxInferBufferRow=10000
  • map.shredding.maxInferBufferMemory=64 mb

Results

Scenario	Regular	Map Shredding	Saved	Saving
Columnar value storage	708,012 bytes	431,637 bytes	276,375 bytes	39.04%
Long hot key storage	40,845,943 bytes	16,365,106 bytes	24,480,837 bytes	59.93%

Scenario Details

• Columnar value storage: key names are short, values follow a repeated pattern with valueRunLength=128 and valueCardinality=4, dictionary encoding enabled. This measures whether promoted hot-key values benefit from columnar and dictionary encoding.
• Long hot key storage: hot key names include 128 bytes of padding, dictionary encoding disabled. This measures the benefit of avoiding repeated long map-key strings in every row.

Conclusion: in this synthetic storage benchmark, map shredding reduces file size in both cases. The biggest gain appears when hot map keys are long and repeated across many rows, saving about 59.93%.

[JingsongLi]

This looks very suitable to be solved using Variant, why not?