Use this skill when designing database schemas, optimizing queries, creating indexes, planning migrations, or choosing between database technologies. Triggers on schema design, normalization, indexing strategies, query optimization, EXPLAIN plans, migrations, partitioning, replication, connection pooling, and any task requiring database architecture or performance decisions.
Use this skill when designing database schemas, optimizing queries, creating indexes, planning migrations, or choosing between database technologies. Triggers on schema design, normalization, indexing strategies, query optimization, EXPLAIN plans, migrations, partitioning, replication, connection pooling, and any task requiring database architecture or performance decisions.
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Database Engineering
A disciplined framework for designing, optimizing, and evolving relational databases in
production. This skill covers schema design, indexing strategies, query optimization,
safe migrations, and operational concerns like connection pooling and partitioning. It is
opinionated about PostgreSQL but most principles apply to any SQL database. The goal is
to help you make the right trade-off at each decision point, not just hand you a syntax
reference.
When to use this skill
Trigger this skill when the user:
Designs a database schema or needs normalization guidance
Asks about creating or tuning indexes (composite, partial, covering)
Wants to understand or optimize a slow query or EXPLAIN plan
Plans a database migration (adding columns, renaming, dropping, backfilling)
Implements soft deletes, audit trails, or temporal data patterns
Sets up connection pooling (PgBouncer, application-level pools)
Partitions a large table by time, hash, or range
Chooses between replication strategies (read replicas, logical replication)
Investigates deadlocks, connection exhaustion, or lock contention
Do NOT trigger this skill for:
NoSQL / document store design (MongoDB, DynamoDB) - different trade-off space
ORM-specific configuration questions unrelated to the underlying SQL
Key principles
Normalize first, then denormalize with a documented reason - Start in third normal
form. Every denormalization must be a conscious decision backed by a measured
performance requirement, not a guess. Write a comment explaining why.
Index for your queries, not your tables - An index that does not serve a query is
write overhead and bloat. Before adding an index, write out the query it serves and
confirm with EXPLAIN ANALYZE that it is actually used.
Migrations must be reversible - Every schema change should have a rollback path.
Use the expand-contract pattern for breaking changes: add the new shape, migrate data,
deprecate the old shape, then drop it in a later release.
Measure before optimizing - EXPLAIN ANALYZE is the ground truth. Never tune a
query without first reading the plan. A query that looks slow may be fast; a query
that looks fast may be causing invisible downstream load.
Plan for growth at schema design time - Ask: "What happens at 100x rows? At 10x
write throughput?" Identify which columns will need indexes, which tables might need
partitioning, and which joins will become expensive before the schema is locked.
Core concepts
Normalization forms
Form
What it eliminates
When to stop here
1NF
Repeating groups, non-atomic columns
Almost never - baseline only
2NF
Partial dependencies on composite keys
Rare - get to 3NF
3NF
Transitive dependencies
Default target for OLTP schemas
BCNF
Remaining anomalies in 3NF edge cases
When you have overlapping candidate keys
Denormalize (with intent) for read-heavy aggregations, pre-computed summaries, or when
JOINs across normalized tables are measured to be a bottleneck.
Index types
Type
Structure
Best for
B-tree
Balanced tree
Equality, range, ORDER BY, IS NULL - the default
Hash
Hash table
Equality-only lookups (rarely faster than B-tree in Postgres)
GIN
Inverted index
JSONB keys, full-text search, array containment
GiST
Generalized search tree
Geometric data, range types, nearest-neighbor
BRIN
Block range index
Very large append-only tables sorted by a natural order (e.g. timestamps)
Composite B-tree indexes follow the leftmost prefix rule: an index on (a, b, c)
serves queries filtering on a, (a, b), or (a, b, c) - but not (b, c) alone.
ACID and WAL
ACID (Atomicity, Consistency, Isolation, Durability) guarantees that transactions are
all-or-nothing, maintain invariants, are isolated from each other, and survive crashes.
PostgreSQL implements these via MVCC (Multi-Version Concurrency Control) - readers
never block writers and vice versa.
WAL (Write-Ahead Log) is the mechanism for durability and replication. Every change
is written to the WAL before it hits the data file. Streaming replication ships WAL
segments to replicas. Logical replication decodes WAL into row-level change events.
Connection pooling
Each PostgreSQL connection is a forked OS process (~5-10 MB RAM). At 500 direct
connections, the database is spending more time on connection overhead than queries.
PgBouncer in transaction mode is the standard solution - it multiplexes many
application connections onto a small pool of server connections. Target 10-20 server
connections per core as a starting point.
Read replicas
Streaming replicas receive WAL in near-real-time (seconds of lag typical, configurable).
Use them to offload analytics, reporting, and read-heavy background jobs. Replication
lag means replicas can return stale data - never send reads that require post-write
consistency to a replica.
Common tasks
Design a normalized schema
Start from an e-commerce domain. Identify entities, attributes, and relationships before
writing DDL.
-- 1. Core entities in 3NFCREATE TABLE customers (
id BIGSERIAL PRIMARY KEY,
email TEXT NOT NULLUNIQUE,
created_at TIMESTAMPTZ NOT NULLDEFAULT now()
);
CREATE TABLE products (
id BIGSERIAL PRIMARY KEY,
sku TEXT NOT NULLUNIQUE,
name TEXT NOT NULL,
price_cents INTNOT NULLCHECK (price_cents >=0)
);
-- 2. Orders reference customers - foreign key with indexCREATE TABLE orders (
id BIGSERIAL PRIMARY KEY,
customer_id BIGINTNOT NULLREFERENCES customers(id),
status TEXT NOT NULLDEFAULT'pending'CHECK (status IN ('pending','confirmed','shipped','cancelled')),
created_at TIMESTAMPTZ NOT NULLDEFAULT now()
);
CREATE INDEX idx_orders_customer_id ON orders(customer_id);
CREATE INDEX idx_orders_status_created ON orders(status, created_at DESC);
-- 3. Junction table for order line itemsCREATE TABLE order_items (
id BIGSERIAL PRIMARY KEY,
order_id BIGINTNOT NULLREFERENCES orders(id),
product_id BIGINTNOT NULLREFERENCES products(id),
quantity INTNOT NULLCHECK (quantity >0),
unit_price_cents INTNOT NULL
);
CREATE INDEX idx_order_items_order_id ON order_items(order_id);
unit_price_cents is intentionally denormalized from products.price_cents. Prices
change over time; the order must record what the customer was charged.
Create effective indexes
-- Composite index: filter first on equality columns, then range/sort-- Serves: WHERE org_id = ? AND status = ? ORDER BY created_at DESCCREATE INDEX idx_orders_org_status_created
ON orders(org_id, status, created_at DESC);
-- Partial index: only index the rows you actually query-- Saves space and stays small even as the table growsCREATE INDEX idx_orders_pending
ON orders(customer_id, created_at)
WHERE status ='pending';
-- Covering index: include non-filter columns to avoid heap fetch-- The query can be answered entirely from the index (index-only scan)CREATE INDEX idx_products_sku_covering
ON products(sku)
INCLUDE (name, price_cents);
-- Check index usage - drop indexes with low scansSELECT schemaname, tablename, indexname, idx_scan, idx_tup_read
FROM pg_stat_user_indexes
ORDERBY idx_scan ASC;
Read and optimize EXPLAIN plans
-- Always use EXPLAIN ANALYZE (BUFFERS) for real execution data
EXPLAIN (ANALYZE, BUFFERS, FORMAT TEXT)
SELECT o.id, c.email, sum(oi.quantity * oi.unit_price_cents)
FROM orders o
JOIN customers c ON c.id = o.customer_id
JOIN order_items oi ON oi.order_id = o.id
WHERE o.status ='pending'GROUPBY o.id, c.email;
Key things to read in the plan output:
Signal
What it means
Action
Seq Scan on a large table
No usable index
Add an index on the filter column
rows=10000 vs actual rows=3
Bad statistics
Run ANALYZE tablename
Hash Join with large Batches
Spilling to disk
Increase work_mem or add index
Nested Loop with large outer set
N+1 at the SQL level
Rewrite as hash join or batch
High Buffers: shared hit
Data in cache - good
No action needed
High Buffers: shared read
Data read from disk
Consider more cache or BRIN index
Write safe migrations
Use the expand-contract pattern for zero-downtime changes:
-- Phase 1 (expand): add nullable column, old code ignores itALTER TABLE orders ADDCOLUMN notes TEXT;
-- Phase 2 (backfill): run in batches to avoid locking
DO $$
DECLARE batch_size INT :=1000;
last_id BIGINT :=0;
BEGIN
LOOP
UPDATE orders
SET notes =''WHERE id > last_id AND id <= last_id + batch_size AND notes ISNULL;
GET DIAGNOSTICS last_id = ROW_COUNT;
EXIT WHEN last_id =0;
PERFORM pg_sleep(0.05); -- yield to avoid lock contention
last_id := last_id + batch_size;
END LOOP;
END $$;
-- Phase 3 (contract): add NOT NULL constraint after all rows are filledALTER TABLE orders ALTERCOLUMN notes SETNOT NULL;
ALTER TABLE orders ALTERCOLUMN notes SETDEFAULT'';
Never ALTER TABLE ... ADD COLUMN ... NOT NULL without a DEFAULT on Postgres < 11. On
Postgres 11+ it is safe only if the default is a constant. On older versions it rewrites
the entire table and takes an exclusive lock.
Implement soft deletes vs hard deletes
-- Soft delete patternALTER TABLE customers ADDCOLUMN deleted_at TIMESTAMPTZ;
-- Partial index keeps active-record queries fastCREATE INDEX idx_customers_active ON customers(email) WHERE deleted_at ISNULL;
-- Application queries always filterSELECT*FROM customers WHERE deleted_at ISNULLAND email = $1;
-- Hard delete with archival (for GDPR / data retention)WITH deleted AS (
DELETEFROM customers WHERE id = $1 RETURNING *
)
INSERT INTO customers_archive SELECT*, now() AS archived_at FROM deleted;
Prefer hard deletes with an archive table for compliance-sensitive data.
Use soft deletes only when you need "undo" semantics or audit trails.
Set up connection pooling
# pgbouncer.ini - transaction mode is best for most web workloads[databases]myapp = host=127.0.0.1 port=5432 dbname=myapp
[pgbouncer]pool_mode = transaction
max_client_conn = 1000; application connections indefault_pool_size = 25; server connections per databasemin_pool_size = 5reserve_pool_size = 5server_lifetime = 3600server_idle_timeout = 600log_connections = 0; disable in high-throughput environments
In transaction mode, prepared statements and SET commands do not persist across
connections. Use DEALLOCATE ALL or disable prepared statements in your driver
(prepared_statement_cache_size=0 in JDBC).
Partition large tables
-- Range partition by month (good for time-series, logs, events)CREATE TABLE events (
id BIGSERIAL,
created_at TIMESTAMPTZ NOT NULL,
type TEXT NOT NULL,
payload JSONB
) PARTITIONBYRANGE (created_at);
CREATE TABLE events_2024_01
PARTITIONOF events FORVALUESFROM ('2024-01-01') TO ('2024-02-01');
CREATE TABLE events_2024_02
PARTITIONOF events FORVALUESFROM ('2024-02-01') TO ('2024-03-01');
-- Automate with pg_partman extensionSELECT partman.create_parent(
p_parent_table =>'public.events',
p_control =>'created_at',
p_type =>'native',
p_interval =>'monthly'
);
-- Partition pruning - Postgres skips partitions outside the WHERE range
EXPLAIN SELECT*FROM events WHERE created_at >='2024-01-15';
-- Should show: Append -> Seq Scan on events_2024_01 (only one child scanned)
Error handling
Error
Root cause
Resolution
deadlock detected
Two transactions acquiring the same locks in opposite order
Enforce a consistent lock acquisition order; use SELECT ... FOR UPDATE SKIP LOCKED for queue patterns
too many connections
App creating connections faster than they close
Add PgBouncer; audit connection pool settings; check for connection leaks
canceling statement due to conflict with recovery
Long query on replica conflicts with WAL replay
Increase max_standby_streaming_delay; move analytics to a dedicated replica
could not serialize access due to concurrent update
SERIALIZABLE isolation write conflict
Retry the transaction; this is expected behavior, not a bug
index bloat / slow index scans
Dead tuples not vacuumed, bloated index pages
Run VACUUM ANALYZE; tune autovacuum_vacuum_scale_factor for high-churn tables
Query slow after data growth
Missing index or stale planner statistics
Run ANALYZE tablename; check with EXPLAIN (ANALYZE, BUFFERS)
Gotchas
ALTER TABLE ... ADD COLUMN ... NOT NULL locks the table on Postgres < 11 - Without a constant DEFAULT, Postgres rewrites the entire table under an exclusive lock. On Postgres 11+ with a constant default it is safe, but a runtime-computed default still triggers a rewrite. Use the expand-contract pattern instead.
Composite index leftmost prefix is strictly enforced - An index on (a, b, c) does not help a query that filters only on b or c. A common mistake is adding an index for a multi-column query and then using it in queries that don't start from the leftmost column. Always verify with EXPLAIN ANALYZE.
PgBouncer transaction mode breaks prepared statements - Many ORMs (Prisma, JDBC) use prepared statements by default. In PgBouncer transaction mode, prepared statements don't persist across connections, causing prepared statement "s1" does not exist errors. Disable prepared statements in your driver (prepared_statement_cache_size=0 for JDBC, pgbouncer_mode: transaction for Prisma).
VACUUM doesn't reclaim disk space by default - Regular VACUUM marks dead tuples as reusable but doesn't shrink the file. Only VACUUM FULL returns disk space to the OS, but it acquires an exclusive lock and rewrites the table. Use pg_repack for online space reclamation on production tables.
Partition pruning only works when the partition key is in the WHERE clause - Joining a partitioned table on a non-partition key column causes Postgres to scan all partitions. Always include the partition column in range queries or the planner cannot prune.
References
For detailed patterns and implementation guidance, load the relevant file from
references/:
references/query-optimization.md - EXPLAIN ANALYZE deep dive, index types, join
strategies, common bottlenecks
Only load a references file if the current task requires it - they are long and will
consume context.
Companion check
On first activation of this skill in a conversation: check which companion skills are installed by running ls ~/.claude/skills/ ~/.agent/skills/ ~/.agents/skills/ .claude/skills/ .agent/skills/ .agents/skills/ 2>/dev/null. Compare the results against the recommended_skills field in this file's frontmatter. For any that are missing, mention them once and offer to install:
