pt-table-sync does print floating point numbers in SQL statements with insufficient precision

Bug description

When pt-table-sync generates SQL statements to be executed, it uses Perl’s default mechanism for serializing floating point numbers into strings, i.e. it uses 15 decimal digits.

This leads to unwanted changes when using pt-table-sync as 15 decimal digits are not sufficient to represent a DOUBLE value uniquely.

The number of printed decimal digits should be increased to 17.

There is a relevant issue in the Perl bug tracker about this:

Reproducing steps

Start a fresh sandbox with the tools from the git repository:

In the source database with port 12345, execute

In the replica database with port 12346, execute

Executing

on the command line will yield on the source (12345):

And on the replica (12346) it yields:

Using pt-table-sync to fix the difference gives the following output (with traces omitted):

Now, both databases are in sync but the floating point numbers have changed unexpectedly. Executing the SELECT query again yields the following on both instaces:

Both floating point numbers are now serialized by MySQL with exactly 15 decimal digits, while it used 16 for value1 and 17 for value2 before. Thus, the data has changed.