Improved replay process #163
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This PR contains an optimization of the procedure for applying changes to Postgresql.
The point is that with a large number of changes on the source (mysql) the replay process may not be able to cope with the flow of incoming events. The situation is exacerbated if the frequently changing replicated table consists of a large number of columns, because there is a block in the query which multiplies all columns of the table with a json object containing all columns with data. This can result in hundreds of gigabytes worth of results which Postgresql has to handle (sort, group, apply some functions). Even high work_mem don't make a significant improvement.
The situation gets even worse if the table has a compound primary key.
This fix eliminates the need to multiply the json object by the number of columns in the table and by the number of rows in the primary key.