Exploring Range Tablet Placement (and Which Split Point Created Each Tablet)

Jim Knicely

In a previous tip, we explored how yb_tablet_metadata makes it easy to see tablet leadership and replica placement across a YugabyteDB cluster.

That works beautifully for hash-sharded tables.

But what about range-sharded tables created with SPLIT AT VALUES?

When you presplit a range table, each split point produces a new tablet. However:

  • ● yb_tablet_metadata does not show human-readable key bounds.

  • ● yb_local_tablets stores the real range boundaries… but in DocDB-encoded form.

In this tip, we’ll:

  • 1. Create an anonymized range-sharded table
  • 2. Presplit it at known UUID boundaries
  • 3. Decode the partition key boundaries
  • 4. Show:
    • ● first / middle / last tablet
    • ● which split window it represents
    • ● which split point produced it
  • 5. Create a reusable helper function that works for any range table

The result is a clean “tablet provenance” view that explains exactly how your splits map to tablets.

Hash vs Range Presplitting Refresher

Hash sharding
				
					CREATE TABLE t (
  id INT,
  PRIMARY KEY(id HASH)
) SPLIT INTO 8 TABLETS;

Hash space is divided evenly. Tablet metadata shows hash ranges.

Range sharding
				
					CREATE TABLE t (
  id INT PRIMARY KEY ASC
) SPLIT AT VALUES ((100), (200), (300));

Here, tablets are divided by actual key boundaries… not hash ranges.

That’s where things get interesting.

Step 1 – Create an Anonymized Range-Sharded Table

This mimics a real customer workload but uses neutral names.

Primary key:

				
					PRIMARY KEY (acct_uuid ASC, event_uuid ASC)

Because the leading column is ASC (not HASH), this is a range-sharded table.

We split on deterministic UUID boundaries:

  • ● acct_id = 250

  • ● acct_id = 500

  • ● acct_id = 750

				
					DROP SCHEMA IF EXISTS demo CASCADE;
CREATE SCHEMA demo;

CREATE TABLE demo.events_r (
  event_uuid     uuid NOT NULL,
  acct_uuid      uuid NOT NULL,
  ext_id         bigint NULL,
  occurred_at    timestamp NOT NULL DEFAULT now(),
  created_at     timestamp NOT NULL DEFAULT now(),
  updated_at     timestamp NOT NULL DEFAULT now(),
  is_archived    boolean NOT NULL DEFAULT false,
  is_active      boolean NOT NULL DEFAULT true,
  note           text,

  CONSTRAINT events_r_pkey
  PRIMARY KEY (acct_uuid ASC, event_uuid ASC)
)
SPLIT AT VALUES
(
  (('00000000-0000-0000-0000-' || lpad(to_hex(250), 12, '0'))::uuid),
  (('00000000-0000-0000-0000-' || lpad(to_hex(500), 12, '0'))::uuid),
  (('00000000-0000-0000-0000-' || lpad(to_hex(750), 12, '0'))::uuid)
);

This produces 4 tablets:

  • 1. (-infinity, 250)

  • 2. [250, 500)

  • 3.[500, 750)

  • 4.[750, +infinity)

Step 2 – Why We Need a Decoder

If we query yb_local_tablets, we see something like:

				
					\x53000100010001000100010001000100010001000100010001000100010001fa00000021

This is not raw UUID bytes.

It is:

  • ● 0x53 → UUID type tag

  • ● UUID bytes encoded in a memcomparable format

  • ● 00 01 → escaped zero byte

  • ● 00 00 → terminator

  • ● ...00000021 → DocDB suffix

So we must unescape the UUID bytes before formatting them.

For a deep dive on decoding other key types (int, timestamp, text, composite keys), see:

Step 3 – UUID Partition Key Decoder

This function handles:

  • ● zero escaping (00 01)

  • ● termination (00 00)

  • ● formatting 16 bytes into UUID

				
					CREATE OR REPLACE FUNCTION yb_decode_uuid_from_pk(pk BYTEA, start_at INT DEFAULT 0)
RETURNS UUID
LANGUAGE plpgsql
IMMUTABLE
AS $$
DECLARE
  i        INT;
  b        INT;
  b2       INT;
  hex_out  TEXT := '';
BEGIN
  IF pk IS NULL OR pk = ''::bytea THEN
    RETURN NULL;
  END IF;

  i := start_at + 1;

  WHILE i < length(pk) LOOP
    b := get_byte(pk, i);

    IF b <> 0 THEN
      hex_out := hex_out || lpad(to_hex(b), 2, '0');
      i := i + 1;
    ELSE
      IF i + 1 >= length(pk) THEN
        EXIT;
      END IF;

      b2 := get_byte(pk, i + 1);

      IF b2 = 0 THEN
        EXIT;
      ELSIF b2 = 1 THEN
        hex_out := hex_out || '00';
        i := i + 2;
      ELSE
        RETURN NULL;
      END IF;
    END IF;

    IF length(hex_out) >= 32 THEN
      EXIT;
    END IF;
  END LOOP;

  IF length(hex_out) < 32 THEN
    RETURN NULL;
  END IF;

  hex_out := left(hex_out, 32);

  RETURN (
    substr(hex_out, 1, 8)  || '-' ||
    substr(hex_out, 9, 4)  || '-' ||
    substr(hex_out, 13, 4) || '-' ||
    substr(hex_out, 17, 4) || '-' ||
    substr(hex_out, 21, 12)
  )::uuid;
END;
$$;
Step 4 – Build the Tablet Provenance View

This query:

  • ● tablet placement

  • ● first / middle / last

  • ● split window

  • ● decoded UUID bounds

  • ● which split point produced the tablet

				
					WITH local_ranges AS (
  SELECT DISTINCT ON (tablet_id)
    tablet_id,
    partition_key_start,
    partition_key_end
  FROM yb_local_tablets
  WHERE namespace_name = current_database()
    AND table_name = 'events_r'
  ORDER BY tablet_id
),
ordered AS (
  SELECT
    lr.*,
    row_number() OVER (ORDER BY partition_key_start NULLS FIRST) AS rn,
    count(*)    OVER ()                                         AS n
  FROM local_ranges lr
),
split_points AS (
  SELECT *
  FROM (VALUES
    (1, (('00000000-0000-0000-0000-' || lpad(to_hex(250), 12, '0'))::uuid)),
    (2, (('00000000-0000-0000-0000-' || lpad(to_hex(500), 12, '0'))::uuid)),
    (3, (('00000000-0000-0000-0000-' || lpad(to_hex(750), 12, '0'))::uuid))
  ) v(split_no, split_acct_uuid)
),
decoded AS (
  SELECT
    o.*,
    yb_decode_uuid_from_pk(o.partition_key_start, 0) AS start_uuid,
    yb_decode_uuid_from_pk(o.partition_key_end,   0) AS end_uuid
  FROM ordered o
)
SELECT
  ytm.tablet_id,
  ytm.leader,

  CASE
    WHEN d.rn = 1   THEN 'first tablet'
    WHEN d.rn = d.n THEN 'last tablet'
    ELSE                 'middle tablet'
  END AS tablet_position,

  CASE
    WHEN d.rn = 1   THEN 'before split #1'
    WHEN d.rn = d.n THEN format('after split #%s', d.n - 1)
    ELSE                 format('between split #%s and split #%s', d.rn - 1, d.rn)
  END AS split_window,

  format('[%s, %s)',
    COALESCE(d.start_uuid::text, '(-infinity)'),
    COALESCE(d.end_uuid::text,   '(+infinity)')
  ) AS acct_uuid_range,

  CASE
    WHEN d.rn = 1 THEN 'implicit (-infinity)'
    ELSE format('split #%s (%s)', sp.split_no, sp.split_acct_uuid)
  END AS produced_by_split_point

FROM yb_tablet_metadata ytm
JOIN decoded d USING (tablet_id)
LEFT JOIN split_points sp
  ON sp.split_acct_uuid = d.start_uuid
WHERE ytm.db_name = current_database()
  AND ytm.relname = 'events_r'
ORDER BY d.rn;

Example output:

				
					.           tablet_id             |     leader     | tablet_position |         split_window          |                               acct_uuid_range                                |             produced_by_split_point
----------------------------------+----------------+-----------------+-------------------------------+------------------------------------------------------------------------------+-------------------------------------------------
 72fd6f70a8c34c8391f94602e53a09e8 | 127.0.0.1:5433 | first tablet    | before split #1               | [(-infinity), 00000000-0000-0000-0000-0000000000fa)                          | implicit (-infinity)
 3ab5f6157cf542fd88bce3150a81c7ed | 127.0.0.1:5433 | middle tablet   | between split #1 and split #2 | [00000000-0000-0000-0000-0000000000fa, 00000000-0000-0000-0000-0000000001f4) | split #1 (00000000-0000-0000-0000-0000000000fa)
 1bf7a1d8c36c4181bcb3b28eeac56f21 | 127.0.0.1:5433 | middle tablet   | between split #2 and split #3 | [00000000-0000-0000-0000-0000000001f4, 00000000-0000-0000-0000-0000000002ee) | split #2 (00000000-0000-0000-0000-0000000001f4)
 3537d8b94ff5475c96d5187e6fbc00cb | 127.0.0.1:5433 | last tablet     | after split #3                | [00000000-0000-0000-0000-0000000002ee, (+infinity))                          | split #3 (00000000-0000-0000-0000-0000000002ee)
(4 rows)

Step 5 – Reusable Helper for Any Range Table

Views cannot take parameters in YSQL, so we create a reusable function.

				
					CREATE OR REPLACE FUNCTION yb_range_tablet_map(p_relname TEXT)
RETURNS TABLE (
  tablet_id TEXT,
  leader TEXT,
  replicas TEXT[],
  tablet_position TEXT,
  split_window TEXT,
  range_start_hex TEXT,
  range_end_hex TEXT,
  partition_key_start BYTEA,
  partition_key_end BYTEA
)
LANGUAGE sql
STABLE
AS $$
WITH local_ranges AS (
  SELECT DISTINCT ON (tablet_id)
    tablet_id,
    partition_key_start,
    partition_key_end
  FROM yb_local_tablets
  WHERE namespace_name = current_database()
    AND table_name = p_relname
  ORDER BY tablet_id
),
ordered AS (
  SELECT
    lr.*,
    row_number() OVER (ORDER BY partition_key_start NULLS FIRST) AS rn,
    count(*)    OVER ()                                         AS n
  FROM local_ranges lr
)
SELECT
  ytm.tablet_id,
  ytm.leader,
  ytm.replicas,

  CASE
    WHEN o.rn = 1     THEN 'first tablet'
    WHEN o.rn = o.n   THEN 'last tablet'
    ELSE                   'middle tablet'
  END AS tablet_position,

  CASE
    WHEN o.rn = 1     THEN 'before split #1'
    WHEN o.rn = o.n   THEN format('after split #%s', o.n - 1)
    ELSE                   format('between split #%s and split #%s', o.rn - 1, o.rn)
  END AS split_window,

  COALESCE(encode(o.partition_key_start, 'hex'), '(empty)') AS range_start_hex,
  COALESCE(encode(o.partition_key_end,   'hex'), '(empty)') AS range_end_hex,

  o.partition_key_start,
  o.partition_key_end
FROM yb_tablet_metadata ytm
JOIN ordered o USING (tablet_id)
WHERE ytm.db_name = current_database()
  AND ytm.relname = p_relname
ORDER BY o.rn;
$$;

Use it like this:

				
					SELECT
  tablet_id,
  leader,
  tablet_position,
  split_window,
  format('[%s, %s)',
    COALESCE(yb_decode_uuid_from_pk(partition_key_start, 0)::text, '(-infinity)'),
    COALESCE(yb_decode_uuid_from_pk(partition_key_end,   0)::text, '(+infinity)')
  ) AS acct_uuid_range
FROM yb_range_tablet_map('demo.events_r')
ORDER BY split_window;

Example output:

				
					.           tablet_id             |     leader     | tablet_position |         split_window          |                               acct_uuid_range
----------------------------------+----------------+-----------------+-------------------------------+------------------------------------------------------------------------------
 3537d8b94ff5475c96d5187e6fbc00cb | 127.0.0.1:5433 | last tablet     | after split #3                | [00000000-0000-0000-0000-0000000002ee, (+infinity))
 72fd6f70a8c34c8391f94602e53a09e8 | 127.0.0.1:5433 | first tablet    | before split #1               | [(-infinity), 00000000-0000-0000-0000-0000000000fa)
 3ab5f6157cf542fd88bce3150a81c7ed | 127.0.0.1:5433 | middle tablet   | between split #1 and split #2 | [00000000-0000-0000-0000-0000000000fa, 00000000-0000-0000-0000-0000000001f4)
 1bf7a1d8c36c4181bcb3b28eeac56f21 | 127.0.0.1:5433 | middle tablet   | between split #2 and split #3 | [00000000-0000-0000-0000-0000000001f4, 00000000-0000-0000-0000-0000000002ee)
(4 rows)

To decode bounds for a specific table, layer a decoder on top.

Final Thoughts

For hash-sharded tables, yb_tablet_metadata is often enough.

For range-sharded tables, the real key boundaries live in yb_local_tablets.

  • When you combine:

    • ● yb_tablet_metadata (placement)

    • ● yb_local_tablets (boundaries)

    • ● decoding logic (from the linked tip)

…you gain full visibility into:

  • ● which split point created each tablet

  • ● how key space is partitioned

  • ● how tablet placement maps to your split strategy

This is incredibly useful when validating presplit strategies before production rollout, especially in multi-tenant or hot-range scenarios.

Have Fun!

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