UNCOMPRESSED_LENGTH(compressed_string)
Returns the length that the compressed string had before being compressed with COMPRESS().
UNCOMPRESSED_LENGTH() returns NULL or an incorrect result if the string is not compressed.
Until MariaDB 10.3.1, returns MYSQL_TYPE_LONGLONG, or bigint(10), in all cases. From MariaDB 10.3.1, returns MYSQL_TYPE_LONG, or int(10), when the result would fit within 32-bits.
SELECT UNCOMPRESSED_LENGTH(COMPRESS(REPEAT('a',30)));
+-----------------------------------------------+
| UNCOMPRESSED_LENGTH(COMPRESS(REPEAT('a',30))) |
+-----------------------------------------------+
| 30 |
+-----------------------------------------------+
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https://mariadb.com/kb/en/uncompressed_length/