GUID/UUID Performance

Why random GUID/UUID keys hurt index performance, and how to mitigate it.

The problem

GUIDs/UUIDs (Globally/Universally Unique Identifiers) are very random. Therefore, INSERTing into an index means jumping around a lot. Once the index is too big to be cached, most INSERTs involve a disk hit. Even on a beefy system, this limits you to a few hundred INSERTs per second.

MariaDB's UUID function.

This blog is mostly eliminated in MySQL 8.0 with the advent of the following function:UUID_TO_BIN(str, swap_flag).

Why it is a problem

A 'standard' GUID/UUID is composed of the time, machine identification and some other stuff. The combination should be unique, even without coordination between different computers that could be generating UUIDs simultaneously.

The top part of the GUID/UUID is the bottom part of the current time. The top part is the primary part of what would be used for placing the value in an ordered list (INDEX). This cycles in about 7.16 minutes.

Some math... If the index is small enough to be cached in RAM, each insert into the index is CPU only, with the writes being delayed and batched. If the index is 20 times as big as can be cached, then 19 out of 20 inserts will be a cache miss. (This math applies to any "random" index.)

Second problem

36 characters is bulky. If you are using that as a PRIMARY KEY in InnoDB and you have secondary keys, remember that each secondary key has an implicit copy of the PK, thereby making it bulky.

It is tempting to declare the UUID VARCHAR(36). And, since you probably are thinking globally, so you have CHARACTER SET utf8 (or utf8mb4). For utf8:

2 - Overhead for VAR
36 - chars
3 (or 4) bytes per character for utf8 (or utf8mb4) So, max length = 2+3*36 = 110 (or 146) bytes. For temp tables 108 (or 144) is actually used if a MEMORY table is used.

To compress

utf8 is unnecessary (ascii would do); but this is obviated by the next two steps
Toss dashes
UNHEX Now it will fit in 16 bytes: BINARY(16)

Combining the problems and crafting a solution

But first, a caveat. This solution only works for "Time based" / "Version 1" UUIDs They are recognizable by the "1" at the beginning of the third clump.

The manual's sample: 6ccd780c-baba-1026-9564-0040f4311e29 . A more current value (after a few years): 49ea2de3-17a2-11e2-8346-001eecac3efa . Notice how the 3rd part has slowly changed over time? Let's data is rearranged, thus:

1026-baba-6ccd780c-9564-0040f4311e29
      11e2-17a2-49ea2de3-8346-001eecac3efa
      11e2-17ac-106762a5-8346-001eecac3efa -- after a few more minutes

Now we have a number that increases nicely over time. Multiple sources won't be quite in time order, but they will be close. The "hot" spot for inserting into an INDEX(uuid) will be rather narrow, thereby making it quite cacheable and efficient.

If your SELECTs tend to be for "recent" uuids, then they, too, will be easily cached. If, on the other hand, your SELECTs often reach for old uuids, they will be random and not well cached. Still, improving the INSERTs will help the system overall.

Code to do it

Let's make Stored Functions to do the messy work of the two actions:

Rearrange fields
Convert to/from BINARY(16)

DELIMITER //

    CREATE FUNCTION UuidToBin(_uuid BINARY(36))
        RETURNS BINARY(16)
        LANGUAGE SQL  DETERMINISTIC  CONTAINS SQL  SQL SECURITY INVOKER
    RETURN
        UNHEX(CONCAT(
            SUBSTR(_uuid, 15, 4),
            SUBSTR(_uuid, 10, 4),
            SUBSTR(_uuid,  1, 8),
            SUBSTR(_uuid, 20, 4),
            SUBSTR(_uuid, 25) ));
    //
    CREATE FUNCTION UuidFromBin(_bin BINARY(16))
        RETURNS BINARY(36)
        LANGUAGE SQL  DETERMINISTIC  CONTAINS SQL  SQL SECURITY INVOKER
    RETURN
        LCASE(CONCAT_WS('-',
            HEX(SUBSTR(_bin,  5, 4)),
            HEX(SUBSTR(_bin,  3, 2)),
            HEX(SUBSTR(_bin,  1, 2)),
            HEX(SUBSTR(_bin,  9, 2)),
            HEX(SUBSTR(_bin, 11))
                 ));

    //
    DELIMITER ;

Then you would do things like

-- Letting MySQL create the UUID:
    INSERT INTO t (uuid, ...) VALUES (UuidToBin(UUID()), ...);

    -- Creating the UUID elsewhere:
    INSERT INTO t (uuid, ...) VALUES (UuidToBin(?), ...);

    -- Retrieving (point query using uuid):
    SELECT ... FROM t WHERE uuid = UuidToBin(?);

    -- Retrieving (other):
    SELECT UuidFromBin(uuid), ... FROM t ...;

Do not flip the WHERE; this will be inefficient because it won't use INDEX(uuid):

WHERE UuidFromBin(uuid) = '1026-baba-6ccd780c-9564-0040f4311e29' -- NO

Wrapup

This shows three thing for speeding up usage of GUIDs/UUIDs:

Shrink footprint (Smaller -> more cacheable -> faster).
Rearrange uuid to make a "hot spot" to improve cachability.
Consider an engine whose indexing suits high-volume random inserts (MyRocks shares some architectural traits which may be beneficial in handling UUIDs, but this is hypothetical and hasn't been tested)

Note that the benefit of the "hot spot" is only partial:

Chronologically ordered (or approximately ordered) INSERTs benefit; random ones don't.
SELECTs/UPDATEs by "recent" uuids benefit; old ones don't benefit.

Postlog

Thanks to Trey for some of the ideas here.

The tips in this document apply to MySQL, MariaDB, and Percona.

Written Oct, 2012.