Debugging Large Data with Rewriter

A customer showed that a particular client reported a less-than-helpful error message when it tried to display some meta-data about a table.

I couldn’t repeat the behavior with just a copy of the schema, so I suspected it was because of the size of data in the customer’s server – somebody had used an int where they needed a long.

The customer’s data was quite large – many hundreds of GB – more than I could easily whip up on my laptop to test. But, I didn’t really need all that data, or even any data at all; I just needed MySQL to pretend it had all that data. Specifically, I needed information_schema to report a large data_length.

Enter Rewriter, the query rewrite plugin that ships with MySQL 5.7 and later.

First, the general query log gave the exact query sent by the client:

select * FROM information_schema.partitions WHERE TABLE_SCHEMA = 'schemaNameHere' AND TABLE_NAME = 'tableNameHere'

Create a copy of that table:

CREATE DATABASE debug_schema;
CREATE TABLE debug_schema.partitions LIKE information_schema.partitions;
INSERT INTO debug_schema.partitions SELECT * FROM information_schema.partitions;

Adjust the copy to report a different value:

UPDATE debug_schema.partitions SET data_length = POW(2, 32);

Install the Rewriter plugin, and add a rule to modify the schema name:

INSERT INTO query_rewrite.rewrite_rules(pattern, replacement) VALUES (
'select * FROM information_schema.partitions WHERE TABLE_SCHEMA = ? AND TABLE_NAME = ?',
'select * FROM debug_schema.partitions WHERE TABLE_SCHEMA = ? AND TABLE_NAME = ?'
);

CALL query_rewrite.flush_rewrite_rules();

Now when I try the client, it thinks information_schema has reported a huge data_length, and I get the same error which my customer had reported. Success!