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Are MySQL UUID inserts really that slow?

📅 2022-10-11

39 days ago

🕑 8 min read

Prevalent Wisdom

Perusing some threads while designing the schema for $CURRENT_JOB, I came across a common refrain that one should never use UUIDs as primary keys.

In this post, I’m going to run the numbers myself and see if things have changed.

UUIDs? Why art thou so slow?

RFC4122 defines a UUID of being constructed as follows:

   0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          time_low                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       time_mid                |         time_hi_and_version   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |clk_seq_hi_res |  clk_seq_low  |         node (0-1)            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         node (2-5)                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Diagram with annotated parts of a UUID
RFC 4122 explained

This makes a v1 UUID “somewhat” sequential. For example in MySQL:

-- Generate 5 UUIDs using the UUID() function
SELECT UUID();
+--------------------------------------+
| UUID()                               |
+--------------------------------------+
| 92734b47-4a21-11ed-984e-0242ac120002 |
| 934ae2ee-4a21-11ed-984e-0242ac120002 |
| 93b2a541-4a21-11ed-984e-0242ac120002 |
| 94470d9e-4a21-11ed-984e-0242ac120002 |
| 948c399b-4a21-11ed-984e-0242ac120002 |
+--------------------------------------+

Which all look pretty similar. The internal UUID() function generates a v1 UUID. MySQL now even has an extra flag on the UUID_TO_BIN() which:

If swap_flag is 1, the format of the return value differs: The time-low and time-high parts (the first and third groups of hexadecimal digits, respectively) are swapped. This moves the more rapidly varying part to the right and can improve indexing efficiency if the result is stored in an indexed column.

But if I were to generate a v4 UUID, I’d get something like:

-- Select 5 UUIDs generated by an external library
SELECT uuid FROM test_table;
+--------------------------------------+
| uuid                                 |
+--------------------------------------+
| 75c67472-d21e-4bb6-92c4-29f628dccfc5 |
| 367ef476-2130-4a23-a8a3-59d9b5ff2230 |
| 9f94ac0e-5ecb-4df3-acc0-d61169d10357 |
| 85812acb-3962-4dff-8fff-988bc57249d2 |
| 88c79d5c-43d3-4b55-ac15-de17b35deb33 |
+--------------------------------------+

Which are all over the place, making the database’s job of inserting them into the table much harder (if used as a primary key).

But why?

How MySQL (InnoDB) stores Primary Keys

For this discussion, I’ll only be considering InnoDB tables, the default storage engine in MySQL 8.0.30. Every time a new row is inserted:

Inserting a row in InnoDB
Inserting a row in InnoDB

This process works great for sequential PKs, ensuring that the new row is inserted at the end of the block. This permits batching the rows for the flush to disk, boosting performance.

However, if the PK is non-sequential, this could potentially be a massive slowdown. If the index is large enough to fit in the RAM, then all inserts are batched and delayed. If not, then every insert would trigger a read from disk. Furthermore, greater the randomness, greater the chance that a block would get flushed before the next insert can occur. Odds are, every insert would require 2 IOPs, one read one write. The chances of having a “hot” page are extremely slim. This is the core problem with a non monotonically increasing PK.

Benchmarks

Test Environment

Test Setup

All tables had just a single uuid column, which was also the primary key. Example:

CREATE TABLE `binary_16` (
  `uuid` binary(16) NOT NULL,
  PRIMARY KEY (`uuid`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci

I tested the insert performance of the following IDs with both v1 and v4 UUIDs (sizes assume 10M rows, without accounting for the PK): | ID Type | Size (MB) | | ------- | --------- | | INT(11) | 38.15 | | BINARY(16) | 152.59 | | VARCHAR(32) | 314.71 | | CHAR(32) | 610.35 | | CHAR(36) | 686.65 |

Note: It’s really hard getting MySQL to trigger a disk I/O, even while inserting ~8,000 inserts/second over 2+ hours. In the end, I had to reduce the RAM available to MySQL to 128M AND reduce the innodb_buffer_pool_size to 64M. It really is a testament to the efficiency of the InnoDB storage engine.

For the actual insert benchmarking, I used sysbench with the following command:

sysbench "$script" --threads=16 --warmup-time=30 --mysql-user=benchmark --mysql-password=password --mysql-db=benchmark --report-interval=1 --time=$1 run
Linode Resource Usage
Finally some disk I/O!

The Lua scripts are available here.

Results

v1 UUIDs

UUIDv1 Scatter Plot
UUIDv1 Scatter Plot

Virtually the same performance between INT(11) AUTO_INCREMENT and CHAR(36) / CHAR(32) / VARCHAR(32) / BINARY(16) columns. So, always use v1 UUIDs, right? Well, not if you need the following:

UUIDv4 Statistics
Mean, median, p95 and p99 QPS for UUIDv1

What about UUIDv4?

v4 UUIDs

The p99 latencies show the real story here. The INT(11) AUTO_INCREMENT PK is ~6x faster than its CHAR(36) equivalent.

UUIDv4 Scatter Plot
UUIDv4 Scatter Plot

The difference starts to become apparent early on. But by the 500s mark (approximately 4M rows) there’s a gap of > 2,000 inserts/second!

UUIDv4 Statistics
Mean, median, p95 and p99 QPS for UUIDv4

What if I run the benchmark for twice the duration?

UUIDv4 Scatter Plot (x2)
UUIDv4 Scatter Plot (x2)

Now the gap widens to nearly 4,000 inserts/second! VARCHAR(32) inserts seemed to fail completely around the 4,300s mark (posibly ran out of storage).

Mean, median, p95 and p99 QPS for UUIDv4 (x2)
Mean, median, p95 and p99 QPS for UUIDv4 (x2)

The p99 difference seems to be even more pronounced (the scale of the Y-axis had to be changed). By this point, there were ~64 million rows in the table.

A Solution

Use an AUTO_INCREMENT primary key with an extra column for the UUID. Something like:

CREATE TABLE `binary_16` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `uuid` binary(16) NOT NULL,
  PRIMARY KEY (`id`),
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci

Now, generate a slugified version of the unique identifier on the application. For example, 75c67472-d21e-4bb6-92c4-29f628dccfc5 with id = 247389 becomes 75c67472-d21e-4bb6-92c4-29f628dccfc5.247389. During fetching, simply split the slugified ID on the . and use the latter part in your WHERE clause. On the returned row, match the requested vs. returned UUIDs. This solution was offered to me by my manager at my previous workplace and has been working out great for us!

What about PK exhaustion?

Use BIGINT. You will not run out.

UUIDs are big!

For storage of the UUIDs, I would recommend using a BINARY(16) data type making use of MySQL’s UUID_TO_BIN() and BIN_TO_UUID() functions.

According to another post on Percona’s Blog:

When the schema is based on UUID values, all these columns and indexes supporting them are char(36). I recently analyzed a UUID based schema and found that about 70 percent of storage was for these values.

A BINARY(16) column would reduce the storage size by 50% (from 36 bytes to 16 bytes). In practice, after accounting for the index it leads to a 77.7% reduction in storage size.

There’s another problem regarding the querying performance of a UUID. An INT can be compared 8 bytes at a time, while a UUID will need to be compared chare-by-char.

Conclusion

Turns out the the insert performance is really bad for v4 UUIDs. If a UUIDv1 fits your requirements, there’s no harm in using it as a primary key. In general, I would recommend against their usage.

This post was an exciting deep dive for me into the inner workings of InnoDB. I’m still just a novice so, please let me know on hi @ this domain if there’s something I missed.

Many thanks to the following folks for their assistance:

References