fix(database): Sync prefixlen of FTS index during restoration (backport #459)

[mergify]

We have started getting this error message while restoring tables with FTS Index :

(1808, "Schema mismatch (Index ft_description field description prefix len 0 doesn't match metadata file value 1))

Root Cause :

MariaDB has some special validation in place for FTS index. It really matters, how the FTS index was added in the database at first place. If we don't follow the same flow to re-create FTS, it will panic.

For example, If we add FTS Index via ALTER and take physical backup of the database

CREATE TABLE articles (
    id INT AUTO_INCREMENT PRIMARY KEY,
    title VARCHAR(200),
    content TEXT
) ENGINE=InnoDB;
ALTER TABLE articles ADD FULLTEXT INDEX ft_content (content);

We can't restore the tablespace ever, if we have created the target table with FTS index with the following syntax

CREATE TABLE articles (
    id INT AUTO_INCREMENT PRIMARY KEY,
    title VARCHAR(200),
    content TEXT,
    FULLTEXT INDEX ft_content (content)
) ENGINE=InnoDB;

SHOW CREATE TABLE ..... will always include FTS Index Creation regardless how it was added.

Solution :

The prefixlen metadata is available inside the .cfg file only (which generates during tablespace export). So, we have to parse the cfg file.

Source : https://github.com/MariaDB/server/blob/d755574c47f101917aee29ea847e214c24c92c0b/storage/innobase/row/row0import.cc#L2816

So, if the FTS indexes prefixlen is 1 according to .cfg file, we have to drop the FTS index and re-create using alter.

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This is an automatic backport of pull request #459 done by Mergify.