Losing PostgreSQL Gains? Blame Inline JSONB!!

The Hidden Cost of JSONB (Inline Storage)

PostgreSQL stores table rows in 8KB pages, packing as many tuples as possible. For a typical row with 10–12 columns, and small text/integers, 40–100 rows can easily fit per page. Typically row count = Page Size(8kb) / row size + row metadata (30-50 bytes approx.)

But the game changes when you add jsonb.

Example

CREATE TABLE events (
    id serial PRIMARY KEY,
    user_id int,
    action text,
    metadata jsonb
);

Suppose metadata which is a jsonb column contains:

{
    "ip": "127.0.0.1",
    "device": "Android",
    "country": "IN"
}

This JSON might be just 100–500 bytes, so PostgreSQL stores it in-line inside the same page (no TOASTing).

Result

• Each row size jumps from ~80 bytes → ~200–400 bytes
• Row count per page drops from 100 → 20–40
• Index scan still needs to read each page for matching rows
• More pages = more I/O, slower performance

Real Benchmark Insight

Performance comparison: Even with a GIN or B-tree index on the JSONB column, PostgreSQL still needs to scan all matching pages to retrieve the full tuple.

Why Index Doesn't Save You

Say you index a JSONB key like:

CREATE INDEX ON events ((metadata->>'ip'));

And query:

SELECT * FROM events WHERE metadata->>'ip' = '127.0.0.1';

PostgreSQL will:

• Use the index to find matching tuples
• Still need to fetch the row from disk
• Because JSONB is in-line, many pages are touched
• More page fetches = more IO = slower queries

What You Can Do

• Force TOAST: Add padding to make JSONB exceed 2KB:

  UPDATE events SET metadata = metadata || jsonb_build_object('padding', repeat('x', 2000));
  

• Split into separate table: If JSONB is rarely queried
• Stick to well defined schema and avoid using jsonb unless absolutely necessary

TL;DR

• JSONB under ~2KB is stored inline
• That bloats each row and reduces rows per page
• More pages scanned = slower indexed reads
• Even efficient indexes can't avoid this penalty
• Force TOAST or redesign if performance matters

Final Thought

Indexes reduce logical lookup cost. But if rows are bloated due to in-line JSONB, you're paying a high physical I/O cost - and that's where PostgreSQL performance dies quietly.

Source Code

You can find the source code and diagram files on GitLab:

👉 rohit yadav / Postgres-JsonB-Performance · GitLab