Read identify columns from iceberg metada

[k-morozov]

Closes: #781

:) SELECT DISTINCT _path FROM datalake."iceberg.foo" WHERE date > '2026-07-13' ;

SELECT
    ProfileEvents['ObjectStorageReadObjects'] AS files_opened_for_read,
    ProfileEvents['S3GetObject'] AS s3_get_object,
    ProfileEvents['S3ReadRequestsCount'] AS s3_get_head,
    ProfileEvents['ReadBufferFromS3Bytes'] AS bytes_read_from_s3,
    ProfileEvents['ParquetMetadataCacheHits'] AS pq_footer_cache_hits,
    ProfileEvents['ParquetMetadataCacheMisses'] AS pq_footer_cache_misses,
    ProfileEvents['ParquetReadRowGroups'] AS row_groups_read,
    ProfileEvents['ParquetPrunedRowGroups'] AS row_groups_pruned,
    ProfileEvents['IcebergPartitionPrunedFiles'] AS files_pruned_by_partition,
    ProfileEvents['IcebergMinMaxIndexPrunedFiles'] AS files_pruned_by_minmax,
    read_rows,
    read_bytes
FROM system.query_log
WHERE (type = 'QueryFinish') AND (query_id = '9780eb0a-d4bb-4f8a-a9ce-f65932f88f88')

# without PR

Row 1:
──────
files_opened_for_read:     7
s3_get_object:             8
s3_get_head:               16
bytes_read_from_s3:        8856
pq_footer_cache_hits:      7
pq_footer_cache_misses:    0
row_groups_read:           7
row_groups_pruned:         0
files_pruned_by_partition: 0
files_pruned_by_minmax:    0
read_rows:                 19
read_bytes:                5010


# with PR

Row 1:
──────
files_opened_for_read:     0
s3_get_object:             1
s3_get_head:               9
bytes_read_from_s3:        8212
pq_footer_cache_hits:      0
pq_footer_cache_misses:    0
row_groups_read:           0
row_groups_pruned:         0
files_pruned_by_partition: 0
files_pruned_by_minmax:    0
read_rows:                 19
read_bytes:                0

Changelog category (leave one):

• Improvement

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Optimize reads filtered on Iceberg identity-partition columns by avoiding reading data files and using the need_only_count fast path.

Documentation entry for user-facing changes

...

CI/CD Options

Exclude tests:

• Fast test
• Integration Tests
• Stateless tests
• Stateful tests
• Performance tests
• Aarch64 tests
• All with ASAN
• All with TSAN
• All with MSAN
• All with UBSAN
• All with Coverage
• All Regression
• Disable CI Cache

Regression jobs to run:

• Fast suites (mostly <1h)
• Aggregate Functions (2h)
• Alter (1.5h)
• Benchmark (30m)
• ClickHouse Keeper (1h)
• Iceberg (2h)
• LDAP (1h)
• OAuth (5m)
• Parquet (1.5h)
• RBAC (1.5h)
• SSL Server (1h)
• S3 (2h)
• S3 Export (2h)
• Swarms (30m)
• Tiered Storage (2h)

[github-actions]

Workflow [PR], commit [89f8e80]

[ianton-ru]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Chef's kiss.

Reviewed commit: 89f8e8017b

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