Migration Patterns

ALTER TABLE Operations

• Add column: ALTER TABLE t ADD COLUMN col Type [DEFAULT expr] [AFTER existing_col]
• Drop column: ALTER TABLE t DROP COLUMN col
• Modify type: ALTER TABLE t MODIFY COLUMN col NewType (must be compatible)
• Rename: ALTER TABLE t RENAME COLUMN old TO new
• These are metadata-only operations — instant for most changes

Engine Changes

• Cannot ALTER engine directly
• Pattern: create new table → insert from old → rename

CREATE TABLE t_new ENGINE = ReplacingMergeTree() ORDER BY id AS SELECT * FROM t_old;
RENAME TABLE t_old TO t_backup, t_new TO t_old;

• For large tables: use INSERT INTO ... SELECT with batching

EXCHANGE TABLES (v22.5+)

• Atomic swap without RENAME chain: EXCHANGE TABLES told AND tnew
• Simpler and safer than RENAME TABLE told TO tbackup, tnew TO told
• Both tables must exist and be in the same database

Zero-Downtime Migrations

1. Create new table with desired schema
2. Create materialized view to capture new inserts: CREATE MATERIALIZED VIEW mv TO tnew AS SELECT ... FROM told
3. Backfill historical data: INSERT INTO tnew SELECT ... FROM told
4. Verify data consistency
5. Switch application to new table
6. Drop old table and materialized view

Data Backfill Patterns

• Batch by partition: INSERT INTO new SELECT * FROM old WHERE toYYYYMM(date) = 202301
• Use maxinsertblocksize and maxthreads for throughput control
• Monitor with system.processes and system.merges
• Verify row counts match after backfill

Lightweight Mutations

• ALTER TABLE t UPDATE col = expr WHERE condition — async by default (mutations_sync = 0)
• Track progress: SELECT * FROM system.mutations WHERE table = 't'
• ALTER TABLE t DELETE WHERE condition — rewrites affected parts
• Throttle impact: set maxrowsper_mutation to limit rows per mutation batch
• Always schedule heavy mutations off-peak; monitor system.mutations for completion

Cross-Server Migration

• Use remote() table function to copy between servers:

INSERT INTO local_db.t SELECT * FROM remote('source_host:9000', 'db', 't', 'user', 'pass')

• For large tables, batch by partition or use clickhouse-local offline approach

clickhouse-local Offline Migrations

• Run migrations without a running server: clickhouse-local --file migration.sql
• Useful for schema changes on cold data or CI/CD validation
• Can operate directly on data files: clickhouse-local -S 'col1 Type1, col2 Type2' --input-format Native < data.bin

Schema Migration Versioning

• Track applied migrations with a dedicated table:

CREATE TABLE _schema_migrations (name String, applied_at DateTime DEFAULT now()) ENGINE = TinyLog;

• Insert a row after each successful migration; check before applying
• Integrate with deployment scripts for idempotent migration runs

Partition Operations

• ALTER TABLE t ATTACH PARTITION id FROM other_table — zero-copy if same structure
• ALTER TABLE t REPLACE PARTITION id FROM other_table — atomic swap
• ALTER TABLE t MOVE PARTITION id TO TABLE other_table — move data

Common Pitfalls

• Nullable to non-Nullable requires default value for existing NULLs
• Changing ORDER BY requires table recreation
• Mutations (UPDATE/DELETE) rewrite all parts — schedule off-peak
• Test migrations on staging with production data volumes
• EXCHANGE TABLES fails if either table is replicated with different replica paths

Postgres → ClickHouse Migration Planning

Advisory only. This section helps you plan a Postgres→ClickHouse migration and recommend the tooling chmonitor already ships — it never executes a migration. Follow the same phased workflow ClickHouse Cloud uses for its managed-Postgres onboarding: discovery → schema design → type mapping → CDC ingestion → validation. Do each phase before the next.

1. Discovery (introspect the source, read-only)

Ground every recommendation in the user's actual schema, not generic advice. Once Postgres connectivity ships (issues #2449/#2451 add a read-only Postgres query path), run these advisory informationschema / pgcatalog queries via that path — they are SELECT-only and safe against a production replica. Until that path exists, hand these to the user to run themselves and paste back.

Tables and row estimates (sizing the migration):

SELECT n.nspname AS schema, c.relname AS table,
       c.reltuples::bigint AS est_rows,
       pg_total_relation_size(c.oid) AS total_bytes
FROM pg_class c JOIN pg_namespace n ON n.oid = c.relnamespace
WHERE c.relkind = 'r' AND n.nspname NOT IN ('pg_catalog','information_schema')
ORDER BY total_bytes DESC;

Columns and types (drives the type-mapping table below):

SELECT table_schema, table_name, column_name, ordinal_position,
       data_type, udt_name, is_nullable,
       numeric_precision, numeric_scale, character_maximum_length
FROM information_schema.columns
WHERE table_schema NOT IN ('pg_catalog','information_schema')
ORDER BY table_schema, table_name, ordinal_position;

Primary keys / unique constraints (drives ORDER BY + dedup strategy):

SELECT tc.table_schema, tc.table_name, tc.constraint_type, kcu.column_name,
       kcu.ordinal_position
FROM information_schema.table_constraints tc
JOIN information_schema.key_column_usage kcu
  ON tc.constraint_name = kcu.constraint_name
 AND tc.table_schema = kcu.table_schema
WHERE tc.constraint_type IN ('PRIMARY KEY','UNIQUE')
  AND tc.table_schema NOT IN ('pg_catalog','information_schema')
ORDER BY tc.table_name, kcu.ordinal_position;

Indexes (candidates for ORDER BY, skip indexes, or projections):

SELECT schemaname, tablename, indexname, indexdef
FROM pg_indexes
WHERE schemaname NOT IN ('pg_catalog','information_schema')
ORDER BY tablename;

Foreign keys (denormalize or JOIN-at-query-time candidates):

SELECT tc.table_name, kcu.column_name,
       ccu.table_name AS foreign_table, ccu.column_name AS foreign_column
FROM information_schema.table_constraints tc
JOIN information_schema.key_column_usage kcu
  ON tc.constraint_name = kcu.constraint_name
JOIN information_schema.constraint_column_usage ccu
  ON ccu.constraint_name = tc.constraint_name
WHERE tc.constraint_type = 'FOREIGN KEY'
  AND tc.table_schema NOT IN ('pg_catalog','information_schema');

2. Type Mapping (Postgres → ClickHouse)

Map every source column to a ClickHouse type. Use the exact source type from the informationschema.columns query above (udtname + numeric_precision/scale).

Nullability: map is_nullable = 'YES' to Nullable(T) only when NULL is semantically meaningful. Nullable() costs a separate null-mask column and disqualifies the column from some optimizations. Prefer a sentinel default (0, '', epoch) for NOT-NULL-in-practice columns; reserve Nullable() for genuine tri-state data.

3. Schema Design Pitfalls (relational → columnar)

ClickHouse is not a drop-in relational target. The biggest planning mistakes:

• PRIMARY KEY → ORDER BY (sorting key), not a uniqueness constraint. A

MergeTree ORDER BY defines the sort/primary-key index; it does not enforce uniqueness. Choose it for query patterns, not because Postgres had a PK there. Order columns low-cardinality → high-cardinality (e.g. ORDER BY (tenantid, eventtype, toStartOfHour(ts), user_id)) so the sparse primary index and granule pruning are effective. A high-cardinality leading column (like a bare uuid PK) makes the index almost useless. See the schema-design-advisor skill for ORDER BY selection detail.

• Unique constraints don't exist. ClickHouse won't reject duplicate rows.

If the source relies on a unique/primary key for dedup, model it with ReplacingMergeTree(version) keyed on that column via ORDER BY, and read with SELECT … FINAL (or argMax/GROUP BY) to collapse duplicates. FINAL caveat: it merges at query time and is expensive on large scans; dedup happens lazily in background merges, so pre-merge reads can still see duplicates. Don't assume "eventually unique" == "unique now."

• B-tree / secondary indexes → skip indexes or projections. There's no

per-row secondary index. For occasional point-lookups on a non-sorting column, add a data-skipping index (INDEX idx col TYPE minmax|set|bloom_filter GRANULARITY n). For a whole alternative access pattern (different ORDER BY), use a projection instead. Most Postgres b-tree indexes should simply be dropped — the sorting key replaces the primary one.

• Foreign keys don't exist and aren't enforced. Two paths: denormalize

(fold the referenced columns into the fact table at ingest — the columnar-DB default, trades storage for JOIN-free reads), or JOIN at query time against a Dictionary / small dimension table (good for small, slowly changing lookups via dictGet). Prefer denormalization for hot analytical paths.

• NULLs and Nullable() cost. As above — every Nullable column carries a

null-mask and blocks some optimizations. Audit which columns truly need it.

• Updates & deletes → CDC-friendly engines, not in-place mutations.

ClickHouse ALTER … UPDATE/DELETE are heavyweight async mutations (see Lightweight Mutations above) — wrong for high-frequency row changes. For a replicated OLTP source, model mutability with the engine: - ReplacingMergeTree(version) — last-write-wins upserts; the CDC version column (e.g. Postgres LSN or updated_at) picks the surviving row. - CollapsingMergeTree(sign) / VersionedCollapsingMergeTree(sign, version) — cancel out old row versions with sign = -1 / +1 pairs; suits delete-heavy or exact-count workloads. Deletes become a tombstone row (sign = -1, or a soft-delete flag) rather than a physical DELETE. This is exactly the shape PeerDB writes (below).

4. CDC Ingestion — use chmonitor's PeerDB integration

Recommend PeerDB. chmonitor already integrates PeerDB as its CDC mechanism — do not invent a Debezium/Kafka-Connect/other-tool HOWTO. PeerDB streams a Postgres source into a ClickHouse destination over logical replication (WAL), and chmonitor already models and monitors these mirrors:

• Mirrors move data between typed peers — DBType.POSTGRES (source) →

DBType.CLICKHOUSE (destination). chmonitor normalizes these peer types in components/peerdb/peerdb-utils.ts (normalizeDbType / dbTypeLabel, the DBTYPEBY_ORDINAL map where 3 = POSTGRES, 8 = CLICKHOUSE).

• A CDC mirror runs the phases chmonitor already visualizes

(PHASE_FLOWS.CDC in peerdb-utils.ts): Setup (replication slot + publication + table init) → Initial snapshot (backfill existing rows) → Snapshot done → CDC streaming (tailing the WAL, ongoing). PeerDB applies inserts/updates/deletes into a ReplacingMergeTree-style destination, which is why the engine choices in §3 matter.

• chmonitor is view-only over an externally-run PeerDB flow-api

(apps/dashboard/src/lib/peerdb/peerdb-config.ts, single instance via PEERDBAPIURL). It reports mirror/flow status; it does not create mirrors or give ad-hoc Postgres SQL access. So the skill's role is to plan the target schema and point the user at the PeerDB pages to run and watch the mirror — not to stand up the pipeline.

• The PeerDB pages live under src/routes/(peerdb)/peerdb/ (peers, mirror,

partitions, logs). Direct the user there to create the Postgres→ClickHouse mirror and monitor snapshot progress + ongoing replication lag.

Managed-cloud analog: on ClickHouse Cloud, ClickPipes (its native Postgres CDC connector) plays the same role as PeerDB — mention it as the managed option for Cloud users, but chmonitor's shipped integration is PeerDB.

5. Validation Checklist

After the initial snapshot completes and CDC is streaming, verify parity before cutover. All checks are read-only:

• Row counts per table. Compare SELECT count() FROM ch_table against the

Postgres SELECT count(*) FROM pg_table (or the discovery reltuples estimate for a fast first pass). Expect small transient drift while CDC is live — recheck at a quiesced point.

• Aggregate / checksum comparison. Beyond counts, compare column-level

aggregates that would catch type-mapping errors: sum()/avg()/min()/max() on numeric columns, count(distinct …) on keys, and per-day/toStartOfDay row-count histograms to catch timezone shifts on timestamptz columns. A matching sum(amount) and max(updated_at) per partition is strong evidence the mapping is correct.

• Replication-lag monitoring via the PeerDB pages. Watch snapshot progress

and steady-state lag on the mirror/partitions pages under src/routes/(peerdb)/peerdb/ (chmonitor surfaces slot size, WAL, and lag — see the pdbFmtLag / pdbFmtBytes formatters). Cut over only when the mirror is STATUS_RUNNING with lag near zero and stable.

• Dedup correctness (ReplacingMergeTree). If the destination uses

ReplacingMergeTree, run comparison counts with FINAL (or argMax) so you compare deduplicated rows, not raw parts still awaiting merge.

Postgres-migration Pitfalls (quick reference)

• Don't copy the Postgres PK as the ClickHouse ORDER BY blindly — order by

cardinality for the read pattern.

• numeric without precision has no exact ClickHouse type — decide String vs

Float64 per column.

• timestamptz needs UTC normalization + a pinned 'UTC' display zone;

convert with toTimeZone at query time.

• No auto-increment — serial/identity becomes a plain integer; new ids are the

app's job.

• Uniqueness/FKs aren't enforced — model dedup with ReplacingMergeTree+FINAL

and denormalize FKs (or use Dictionary JOINs).

• Recommend PeerDB (chmonitor's shipped CDC), ClickPipes as the Cloud analog —

not a generic CDC stack.