Imported from personamanagmentlayer/pcl (stdlib/data/analytical-databases-expert/SKILL.md). Install upstream with npx skills add personamanagmentlayer/pcl --skill analytical-databases-expert. Copyright stays with the author.

Analytical Databases Expert

Columnar engines answer aggregate questions over large tables. They are not faster row stores — they trade single-row access for the ability to scan one column of a billion rows without touching the rest.

Core Concepts

Why Columnar Wins

Storing values of one column contiguously means a query reads only the columns it names, and values of one type compress together far better than mixed rows. SELECT avg(amount) FROM events touches one column; the row store reads every byte of every row.

The corollary is the cost: fetching one complete row means reassembling it from every column, and single-row updates are expensive or unsupported. Use a row store for transactions and a columnar store for analysis. Trying to make one system do both well is the usual architectural mistake.

The Data Is Often the Format

With Parquet, storage and engine are separable. Files on object storage, queried by whatever engine suits: DuckDB locally, Spark for scale, the warehouse for governed access. Choosing the format well matters more than choosing the engine.

Selection Is About Deployment, Not Speed

DuckDB is the default worth trying first: most "big data" is under a terabyte, and a single node handles it.

DuckDB

Runs in-process. No server, no cluster, and it queries Parquet, CSV and JSON on local disk or object storage directly.

import duckdb

con = duckdb.connect("analytics.duckdb")

# Query files directly; no load step
con.sql("""
    SELECT date_trunc('month', order_date) AS month,
           region,
           sum(amount_minor) / 100.0 AS revenue,
           count(*)                   AS orders
    FROM read_parquet('s3://bucket/orders/year=*/month=*/*.parquet')
    WHERE order_date >= DATE '2026-01-01'
    GROUP BY ALL
    ORDER BY month, region
""").show()

GROUP BY ALL groups by every non-aggregated column — a small ergonomic win that removes a common source of error.

What makes DuckDB useful in practice:

-- Predicate and projection pushdown into Parquet, including partition pruning
SELECT count(*) FROM read_parquet('data/**/*.parquet', hive_partitioning = true)
WHERE year = 2026 AND region = 'EU';

-- Query Postgres directly, join it to files
INSTALL postgres; LOAD postgres;
ATTACH 'dbname=shop host=db.internal' AS pg (TYPE postgres, READ_ONLY);
SELECT c.segment, sum(o.amount_minor)
FROM read_parquet('s3://bucket/orders/*.parquet') o
JOIN pg.public.customers c ON c.id = o.customer_id
GROUP BY 1;

-- Write partitioned output
COPY (SELECT * FROM events) TO 'out/'
  (FORMAT PARQUET, PARTITION_BY (year, month), COMPRESSION ZSTD);

Constraints to respect: one writer process at a time, and memory bounded by the machine. Set SET memory_limit='8GB' and SET temp_directory='/var/tmp/duckdb' so large joins spill instead of failing.

ClickHouse

Built for high-rate ingest and sub-second aggregation. The table definition carries most of the performance decisions.

CREATE TABLE events (
    event_time  DateTime64(3, 'UTC'),
    tenant_id   UInt64,
    user_id     UInt64,
    event_type  LowCardinality(String),
    country     LowCardinality(String),
    amount      Decimal(18, 4),
    properties  Map(String, String)
)
ENGINE = MergeTree
PARTITION BY toYYYYMM(event_time)
ORDER BY (tenant_id, event_type, event_time)
TTL event_time + INTERVAL 2 YEAR DELETE
SETTINGS index_granularity = 8192;

The three decisions that determine everything:

• ORDER BY is the primary index. Put the columns you filter on first, in decreasing selectivity. A query that does not filter on the leading column scans the whole partition.
• PARTITION BY is for data management, not query speed — dropping a month is instant. Keep partitions coarse; thousands of small partitions degrade merges badly.
• LowCardinality(String) for columns with few distinct values gives large compression and speed gains.

-- Pre-aggregate what dashboards ask for repeatedly
CREATE MATERIALIZED VIEW events_daily
ENGINE = SummingMergeTree
ORDER BY (tenant_id, day, event_type)
AS SELECT tenant_id, toDate(event_time) AS day, event_type,
          count() AS events, sum(amount) AS amount
FROM events GROUP BY tenant_id, day, event_type;

Insert in large batches — thousands to a million rows — never row by row. Each insert creates a part, and too many parts is the most common way to make a ClickHouse cluster unhealthy.

Updates and deletes are asynchronous mutations that rewrite parts. Design so you rarely need them: append events, and correct with a ReplacingMergeTree or a sign column rather than ALTER TABLE … UPDATE.

File Formats

Parquet layout decisions that matter:

• File size 128 MB – 1 GB. Thousands of small files make listing and planning dominate query time; a single huge file prevents parallelism.
• Partition on what you filter, usually date, and keep cardinality low. Partitioning by user id creates millions of directories.
• Sort within files by the column you filter on, so row-group statistics allow skipping.
• ZSTD compression — better ratio than Snappy at comparable speed for analytics.

Query Patterns

-- Aggregate first, then join. Never join large-to-large and then aggregate.
WITH monthly AS (
  SELECT customer_id, date_trunc('month', order_date) AS month,
         sum(amount_minor) AS revenue
  FROM orders WHERE order_date >= DATE '2026-01-01'
  GROUP BY 1, 2
)
SELECT c.segment, m.month, sum(m.revenue) / 100.0 AS revenue
FROM monthly m JOIN customers c ON c.id = m.customer_id
GROUP BY 1, 2;

-- Approximate counting is orders of magnitude cheaper and usually adequate
SELECT approx_count_distinct(user_id) FROM events WHERE day >= today() - 30;

Read the plan rather than guessing:

EXPLAIN ANALYZE SELECT …;                       -- DuckDB
EXPLAIN indexes = 1 SELECT …;                   -- ClickHouse: parts and granules read

The number to watch is bytes scanned. In a warehouse it is also the bill.

Cost Control

For metered warehouses, cost is a function of bytes scanned and compute time:

• Always filter the partition column. A missing date predicate scans history.
• Never SELECT * — you pay per column read.
• Materialise repeated aggregations rather than recomputing per dashboard load.
• Set query and per-user limits, and alert on the top spenders.
• Auto-suspend idle warehouses; separate ELT compute from BI compute so a heavy transform does not block analysts.
• Sample during development. TABLESAMPLE or a LIMIT-ed CTE while iterating on a query.

Best Practices

• Model for the questions. Star schemas remain right: a fact table plus conformed dimensions, denormalised enough to avoid deep join chains.
• Types are compression. Integers over strings, LowCardinality or dictionary encoding for enums, dates as dates.
• Money as integer minor units or Decimal, never floating point.
• Ingest idempotently with a natural key or a batch id, so a replayed load does not double-count.
• Keep raw data. Storage is cheap; a reprocessable landing zone lets you fix transformation bugs retroactively.
• Test transformations with fixed inputs and expected outputs, like any code.

Anti-Patterns

• Using an OLAP engine as an OLTP store — single-row updates and point reads are what it is worst at.
• Small-file proliferation — compact on a schedule.
• Over-partitioning — one partition per user id or per hour at low volume.
• SELECT * in a metered warehouse.
• Row-by-row inserts into ClickHouse — the fastest route to an unhealthy cluster.
• ORDER BY columns nobody filters on — the index earns nothing.
• Unbounded retention — define a TTL at table creation, not after the bill.

Reference Documentation

• Modelling and Pipelines — dimensional modelling, incremental loads, slowly changing dimensions, data quality tests, and moving between engines

Resources

• DuckDB documentation
• ClickHouse documentation
• Apache Parquet format
• Ralph Kimball, The Data Warehouse Toolkit (3rd ed.)
• dbt documentation — see also the dbt-expert skill