ClickHouse Study Flashcards
Column-oriented open-source relational database built for online analytical processing (OLAP). Click any card to flip and test your knowledge.
What is MergeTree in ClickHouse?
The default and most powerful table engine family, featuring primary key indexing, partition pruning, and background data merging.
Why is ClickHouse column-oriented?
Columnar storage reads only queried columns from disk, dramatically accelerating analytical aggregation queries over billions of rows.
How does Primary Key work in ClickHouse MergeTree?
Primary keys do not enforce row uniqueness; instead, they create a sparse index pointing to data mark ranges.
What is ReplacingMergeTree?
A table engine that asynchronously deduplicates rows matching the primary key in background merges.
What is SummingMergeTree?
A specialized table engine that automatically sums numeric columns sharing the same primary key during background merges.
What is vector execution in ClickHouse?
Processing data in memory blocks (vectors) using SIMD CPU instructions for high query execution performance.
What is a Materialized View in ClickHouse?
Stores transformed rows continuously as data is inserted into the source table, avoiding query-time computation.
What does arrayJoin() do?
It expands array elements into individual result output rows, similar to MongoDB's $unwind.
How does Partition Pruning work?
Skipping unneeded data partition files based on WHERE conditions matching the PARTITION BY expression.
What is the system.parts table?
A system catalog table providing metrics about physical MergeTree table parts, sizes, and row counts.
Need practical SQL queries for ClickHouse?
Explore our ready-to-use copy-paste query recipes in the Cookbook.