PostgreSQL Performance & Query Optimization · Lesson

TOAST Internals and Large Value Storage

Understand how PostgreSQL stores oversized columns and tune compression and external storage thresholds.

TOAST Internals and Large Value Storage is a free PostgreSQL Performance & Query Optimization lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the PostgreSQL Performance & Query Optimization learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Why TOAST Exists

PostgreSQL stores rows on fixed-size 8 KB pages. A single row cannot span multiple pages, so a wide value (a long text, big jsonb, or bytea) would never fit.

TOAST (The Oversized-Attribute Storage Technique) solves this by compressing oversized columns and, if still too large, slicing them into chunks stored in a separate side table.

Keeps the main heap row small and cache-friendly.
Lets a logical value far exceed 8 KB (up to ~1 GB).
Happens automatically and transparently to your queries.

The TOAST Threshold

TOAST kicks in when a row's total size would exceed TOAST_TUPLE_THRESHOLD, which is 2 KB (one quarter of the 8 KB page) by default.

When that limit is crossed, PostgreSQL compresses and/or moves the largest toastable attributes out of line until the row fits under TOAST_TUPLE_TARGET (also ~2 KB).

Only columns of variable-length types (text, varchar, jsonb, bytea, arrays, etc.) are toastable. Fixed-width types like integer or timestamptz are never toasted.

Finding the TOAST Table

Every table with at least one toastable column gets an associated TOAST table named pg_toast.pg_toast_<oid>. You can locate it from the catalog.

The reltoastrelid column links a heap relation to its TOAST relation; a value of 0 means no TOAST table was created.

SELECT c.relname,
       c.reltoastrelid,
       t.relname AS toast_table
FROM pg_class c
LEFT JOIN pg_class t ON t.oid = c.reltoastrelid
WHERE c.relname = 'documents';

The Four Storage Strategies

Each column has a storage strategy controlling whether it can be compressed and/or moved out of line:

PLAIN — no compression, no out-of-line; only valid for non-toastable types.
EXTENDED — allow both compression and out-of-line storage (default for most varlena types).
EXTERNAL — allow out-of-line but no compression (faster substring access).
MAIN — allow compression but keep in the main table unless absolutely necessary.

Inspecting Per-Column Storage

Query pg_attribute.attstorage to see each column's strategy. The codes map to: p=PLAIN, e=EXTERNAL, m=MAIN, x=EXTENDED.

SELECT attname,
       atttypid::regtype AS type,
       CASE attstorage
         WHEN 'p' THEN 'plain'
         WHEN 'e' THEN 'external'
         WHEN 'm' THEN 'main'
         WHEN 'x' THEN 'extended'
       END AS storage
FROM pg_attribute
WHERE attrelid = 'documents'::regclass
  AND attnum > 0
  AND NOT attisdropped;

Changing a Column's Strategy

Use ALTER TABLE ... SET STORAGE to override the default. A common optimization: if you frequently read random substrings of a large bytea (e.g. range reads on a blob), switch to EXTERNAL so values are stored uncompressed and can be partially fetched without decompressing the whole thing.

The new strategy applies only to rows written after the change; existing data is not rewritten until updated.

ALTER TABLE documents
  ALTER COLUMN payload SET STORAGE EXTERNAL;

-- Force a rewrite to apply it to existing rows:
VACUUM FULL documents;

Compression Algorithms: pglz vs lz4

PostgreSQL compresses TOAST values before considering out-of-line storage. Two algorithms are available:

pglz — the historic built-in, decent ratio, slower.
lz4 — available since PostgreSQL 14, much faster compress/decompress with a slightly lower ratio. Requires the server to be built with lz4 support.

The cluster-wide default is set by default_toast_compression.

SHOW default_toast_compression;

-- Set lz4 cluster-wide (postgresql.conf or per session):
SET default_toast_compression = 'lz4';

Per-Column Compression

From PostgreSQL 14 onward you can set the compression method on individual columns with SET COMPRESSION. This is independent of the storage strategy.

Choose lz4 for hot, large columns where CPU during read/write matters; keep pglz or use EXTERNAL when ratio or substring access dominates.

ALTER TABLE documents
  ALTER COLUMN body SET COMPRESSION lz4;

-- Inspect chosen method per column:
SELECT attname, attcompression
FROM pg_attribute
WHERE attrelid = 'documents'::regclass
  AND attnum > 0;

Tuning the Out-of-Line Target

The reloption toast_tuple_target controls how aggressively PostgreSQL pushes attributes out of line: it sets the size the main tuple is shrunk toward (valid range 128 bytes to ~8160).

Lowering it makes more values go to TOAST sooner, keeping the main heap dense and improving scan performance when the big columns are rarely read. Raising it keeps more data inline.

ALTER TABLE documents
  SET (toast_tuple_target = 512);

-- Verify current reloptions:
SELECT reloptions
FROM pg_class
WHERE relname = 'documents';

Measuring TOAST Footprint

Use the size functions to separate main-table bytes from TOAST bytes. This tells you whether your bloat lives in the heap or in oversized values.

pg_table_size — heap + TOAST + their indexes' TOAST, excluding regular indexes.
pg_relation_size(rel, 'main') — just the main fork.

SELECT
  pg_size_pretty(pg_relation_size('documents'))        AS heap,
  pg_size_pretty(
    pg_total_relation_size(reltoastrelid)
  )                                                     AS toast,
  pg_size_pretty(pg_total_relation_size('documents'))  AS total
FROM pg_class
WHERE relname = 'documents';

Practical Performance Implications

TOAST is invisible until it isn't. Key effects to remember:

Reading a toasted column triggers extra index lookups into the TOAST table and possible decompression — avoid SELECT * when you only need small columns.
Values stay TOASTed across UPDATEs that don't touch them, so unrelated updates are cheap.
EXTERNAL enables efficient substr() / range reads on large uncompressed blobs.
Switching pglz to lz4 can cut write CPU dramatically on insert-heavy large-value workloads.

Quick Check

You have a table whose large bytea column is read mostly via substr() on small byte ranges, and these reads are slow because each access decompresses the entire value. Which single change best fixes this?

Recap

You learned how PostgreSQL handles oversized values:

TOAST triggers when a row would exceed the ~2 KB threshold, compressing then moving large varlena columns into pg_toast.* tables.
Four strategies — PLAIN, MAIN, EXTENDED (default), EXTERNAL — control compression and out-of-line placement, set via ALTER TABLE ... SET STORAGE.
lz4 (PG14+) offers faster compression than pglz; choose per column with SET COMPRESSION or cluster-wide via default_toast_compression.
toast_tuple_target tunes how eagerly values leave the heap; size functions reveal how much storage lives in TOAST.
Pick EXTERNAL for substring/range reads, lz4 for write-heavy large values, and avoid SELECT * to skip needless detoasting.

Frequently asked questions

Is the “TOAST Internals and Large Value Storage” lesson free?

Yes — the full text of “TOAST Internals and Large Value Storage” is free to read here on the web, and the PostgreSQL Performance & Query Optimization course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the PostgreSQL Performance & Query Optimization course, upgrade to CoddyKit PRO.

What will I learn in “TOAST Internals and Large Value Storage”?

Understand how PostgreSQL stores oversized columns and tune compression and external storage thresholds. You practise PostgreSQL Performance & Query Optimization with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start PostgreSQL Performance & Query Optimization?

No prior experience is required. PostgreSQL Performance & Query Optimization on CoddyKit is structured for beginners through advanced learners; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “TOAST Internals and Large Value Storage” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this PostgreSQL Performance & Query Optimization lesson?

Yes. Every PostgreSQL Performance & Query Optimization lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

Measuring Table and Index Bloat Accurately
Reclaiming Space with pg_repack
Tuning Fillfactor for Update-Heavy Tables
TOAST Internals and Large Value Storage

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