Core: Cache PartitionData template in PartitionsTable to avoid rebuilding Avro schema per partition

[Wenjun7J]

What is changed

This change avoids rebuilding the same PartitionData Avro schema for every partition row when scanning the partitions metadata table.

Instead of creating a fresh PartitionData(partitionType) for each partition value, PartitionsTable now creates one PartitionData template per scan and reuses it through copyFor(key).

A regression test is also added to verify that partition rows produced within the same scan reuse the same underlying Avro schema instance.

Why

PartitionsTable currently constructs partition rows like this:

• create PartitionData(partitionType)
• convert partition type to Avro schema
• copy the partition key into the new object

When a table has many partition values, this repeats the same schema conversion over and over again, creating heavy allocation pressure in:

• PartitionData.partitionDataSchema
• AvroSchemaUtil.convert
• TypeToSchema$WithTypeToName.struct

This is especially visible for wide partition specs and large metadata table scans.

External reproduction

Used a standalone repro app that scans the Iceberg partitions metadata table for a table with:

• 20,000 partition values
• 4 partition columns
• repeated full partitionsTable scans

         try (CloseableIterable<FileScanTask> tasks = partitionsTable.newScan().planFiles()) {
                for (FileScanTask task : tasks) {
                    try (CloseableIterable<StructLike> rows = task.asDataTask().rows()) {
                        for (StructLike row : rows) {
                            StructProjection partitionData = row.get(0, StructProjection.class);
                            if (partitionData == null) {
                                throw new IllegalStateException("Partition row returned null partition data");
                            }
                            partitionRows++;
                        }
                    }
                }
            }

Before fix (origin/main)

• Average wall clock time: 12.71s
• Average max RSS: 5,938,604 KB (~5.66 GiB)

After fix

• Average wall clock time: 5.24s
• Average max RSS: 1,483,155 KB (~1.41 GiB)

Improvement

• Wall clock time reduced by 58.8%
• Max RSS reduced by 75.0%

[Wenjun7J]

@RussellSpitzer @pvary could you please take a look?