MySQL中文件排序中单双路排序
在执行相关的SQL语句中，在mysql里进程容易出现一些file temp类似的字眼，该字眼告诉我们我们的宿主sql采用了文件排序等操作，这可能会极大的影响我们的需要获取结果集的效率问题。
mysql中有2中文件排序算法:单路排序和双路排序;他们的区别于MYSQL中的max_length_for_sort_date具有相关性；单路算法的排序可以为排序中的每一行创建固定的缓冲区；如果库中出现超大字符串的时候，比如；BLOB,TEXT等可以采用前缀排序的算法，但是其值不能超过参数规定的值大小，在生产环境下，建议采用更多的是单路的排序算法，他可以使磁盘的读写效率更高，sort_merge_passes的值会加大等！以下是相关innodb中给出的提示；
ut_ad(error == DB_SUCCESS);
/* Commit the data dictionary transaction in order to release
the table locks on the system tables. This means that if
MySQL crashes while creating a new primary key inside
row_merge_build_indexes(), indexed_table will not be dropped
by trx_rollback_active(). It will have to be recovered or
dropped by the database administrator. */
trx_commit_for_mysql(trx);
row_mysql_unlock_data_dictionary(trx);
dict_locked = FALSE;
ut_a(trx->n_active_thrs == 0);
ut_a(UT_LIST_GET_LEN(trx->signals) == 0);
if (UNIV_UNLIKELY(new_primary)) {
/* A primary key is to be built. Acquire an exclusive
table lock also on the table that is being created. */
ut_ad(indexed_table != prebuilt->table);
error = row_merge_lock_table(prebuilt->trx, indexed_table,
LOCK_X);
if (UNIV_UNLIKELY(error != DB_SUCCESS)) {
goto error_handling;
}
}
/* Read the clustered index of the table and build indexes
based on this information using temporary files and merge sort</span>. */
error = row_merge_build_indexes(prebuilt->trx,
prebuilt->table, indexed_table,
index, num_of_idx, table); <span style="color: rgb(51, 102, 255);">----handler0alter.cc指定句柄操作过程中文件排序带来的merge的操作
for (i = 0; i < n_index; i++) {
row_merge_buf_t* buf = merge_buf[i];
merge_file_t* file = &files[i];
const dict_index_t* index = buf->index;
if (UNIV_LIKELY
(row && row_merge_buf_add(buf, row, ext))) {
file->n_rec++;
continue;
}
/* The buffer must be sufficiently large
to hold at least one record. */
ut_ad(buf->n_tuples || !has_next);
/* We have enough data tuples to form a block.
Sort them and write to disk. */
if (buf->n_tuples) {
if (dict_index_is_unique(index)) {
row_merge_dup_t dup;
dup.index = buf->index;
dup.table = table;
dup.n_dup = 0;
row_merge_buf_sort(buf, &dup);<span style="color: rgb(51, 102, 255);">---row0merge.c文件中的，merge算法中排序的指定