MySQL 5.7 Release Notes - Download - MySQL

MySQL 5.7 Release Notes• The modified filesort algorithm now includes an additional optimization designed to enable moretuples to fit into the sort buffer: For additional columns of type CHAR or VARCHAR, or any nullablefixed-size data type, the values are packed. For example, without packing, a VARCHAR(255) columnvalue containing only 3 characters takes 255 characters in the sort buffer. With packing, the valuerequires only 3 characters plus a two-byte length indicator.For data containing packable strings shorter than the maximum column length or many NULLvalues, more records fit into the sort buffer. This improves in-memory sorting of the sort buffer andperformance of disk-based merge sorting of the temporary file.In edge cases, packing may be disadvantageous: If packable strings are the maximum columnlength or there are few NULL values, the space required for the length indicators reduces the numberof records that fit into the sort buffer and sorting is slower in memory and on disk.Packing is not applicable if the filesort uses a priority queue for sorting, as is the case when anORDER BY ... LIMIT optimization is applied (see Optimizing LIMIT Queries).If a filesort is done, optimizer trace output includes a filesort_summary block. For example:"filesort_summary": {"rows": 100,"examined_rows": 100,"number_of_tmp_files": 0,"sort_buffer_size": 25192,"sort_mode": ""}The sort_mode value provides information about the algorithm used and the contents of the sortbuffer:: sort using row pointers: sort using additional fields: sort using packed additional fieldsFor additional information about the filesort algorithm, see ORDER BY Optimization. Forinformation about the optimizer trace, see MySQL Internals: Tracing the Optimizer.Packaging Notes• Previously, MySQL Server distributions included the MySQL Reference Manual in Info format (theDocs/mysql.info file). Because the license for the manual restricts redistribution, its inclusion inCommunity packages caused problems for downstream redistributors, such as those who createLinux distributions. Community distributions of MySQL Server no longer include the mysql.info file,to make the repackaging and redistribution process easier (for example, the source tarball and itschecksum can be used directly). This change applies to all source and binary Community packagingformats. Commercial (Enterprise) distributions are unchanged.For those who wish to continue using the MySQL Reference Manual in Info format, we have made itavailable at http://dev.mysql.com/doc/.Performance Schema Notes• The Performance Schema now exposes metadata lock information:• Locks that have been granted (shows which sessions own which current metadata locks)• Locks that have been requested but not yet granted (shows which sessions are waiting for whichmetadata locks).• Lock requests that have been killed by the deadlock detector or timed out and are waiting for therequesting session's lock request to be discarded39