1. Insert writes a part
Each INSERT writes one new immutable part on disk.
INSERT INTO events VALUESTyporithm · medium
Parts & Merges
parts-and-mergesclickhousepartsmergeswrite-path
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Full lesson · 8 learning steps Open any exercise for focused practice
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Step 1 of 8 · intro card
Learn Parts & Merges
Columns & Engines
Each INSERT writes a new immutable part; ClickHouse merges parts in the background, and OPTIMIZE ... FINAL forces a merge.
MergeTree never updates rows in place — every INSERT creates a new sorted, immutable part.
A background process continuously merges small parts into larger ones, reclaiming space and keeping reads fast.
OPTIMIZE TABLE ... FINAL forces an immediate full merge, but it is expensive and meant for occasional maintenance.
01
INSERT = one new immutable part.
02
Background merges combine parts.
03
OPTIMIZE FINAL forces a merge (costly).
INSERT INTO events VALUES
('2026-06-01', 42, 'click', 1.5),
('2026-06-01', 42, 'view', 0.0);
OPTIMIZE TABLE events FINAL;Read the implementation
Connect each part of the code to the concept before you type.
2. Batch the rows
Send many rows per INSERT to avoid creating too many tiny parts.
('2026-06-01', 42, 'click', 1.5),
('2026-06-01', 42, 'view', 0.0)3. Force a merge
FINAL forces parts to merge now — costly, use rarely.
OPTIMIZE TABLE events FINALA complete learning loop
01
Understand
Read the concept map and follow the code breakdown.
02
Reconstruct
Type the implementation, repair mistakes and work through every recall exercise.
03
Verify
Use the final check without hints; revisit weak concepts in short practice sessions.