25.8.15 Backport of #70775: Add feature to impersonate/proxy user sessions

[zvonand]

Changelog category (leave one):

• New Feature

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

Added new SQL statement EXECUTE AS to support user impersonation (ClickHouse#70775 by @shiyer7474, ClickHouse#90437 by @alexbakharew)

CI/CD Options

Exclude tests:

• Fast test
• Integration Tests
• Stateless tests
• Stateful tests
• Performance tests
• All with ASAN
• All with TSAN
• All with MSAN
• All with UBSAN
• All with Coverage
• All with Aarch64
• 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)
• Parquet (1.5h)
• RBAC (1.5h)
• SSL Server (1h)
• S3 (2h)
• Tiered Storage (2h)

[github-actions]

Workflow [PR], commit [3aea4d1]

[zvonand]

S3 fails: Altinity/clickhouse-regression#93

Aggregate fails: uniqTheta was updated in upstream, not related, test will be updated separately by QA team

[DimensionWieldr]

CI Failure Analysis

1. Regression Tests (aggregate_functions_1, S3 suites) - Pre-existing Failures

Failures: uniqTheta, zero copy replication

Evidence these are pre-existing on base branch (releases/25.8.15) MasterCI #21579463803:

• AggregateFunctions (1) = failure (x86_64 + aarch64)
• S3 (aws_s3, 2) = failure (x86_64 + aarch64)
• S3 (azure, 2) = failure (x86_64 + aarch64)
• S3 (minio, 2) = failure (x86_64 + aarch64)

Verdict: Pre-existing failures on base branch. Unrelated to PR changes.

---

2. Stateless tests (amd_asan, flaky check) - Infrastructure

Failure: failure: Start ClickHouse Server

Verdict: CI infrastructure issue during server initialization. Unrelated to PR changes.

---

Summary

Test Category	Failure Type	Related to PR?
Regression (8 jobs)	Pre-existing failures	No
Flaky check	Server startup	No

Verdict: All failures are unrelated to the PR changes.