InnoDB: Reduce memcpy() load in row_sel_pop_cached_row_for_mysql()

by copying only a prefix of the row that covers the requested
columns.


innobase/include/row0mysql.h:
  row_prebuilt_struct: Add field mysql_prefix_len
  in order to reduce memcpy() time in
  row_sel_pop_cached_row_for_mysql().
innobase/row/row0sel.c:
  row_sel_pop_cached_row_for_mysql(): memcpy() only
  mysql_prefix_len bytes instead of mysql_row_len.
sql/ha_innodb.cc:
  build_template(): Initialize prebuilt->mysql_prefix_len.

innobase/include/row0mysql.h

@@ -599,6 +599,8 @@ struct row_prebuilt_struct {
 					that was decided in ha_innodb.cc,
 					::store_lock(), ::external_lock(),
 					etc. */
+	ulint		mysql_prefix_len;/* byte offset of the end of
+					the last requested column */
 	ulint		mysql_row_len;	/* length in bytes of a row in the
 					MySQL format */
 	ulint		n_rows_fetched;	/* number of rows fetched after

innobase/row/row0sel.c

@@ -2849,8 +2849,9 @@ row_sel_pop_cached_row_for_mysql(
 	mysql_row_templ_t*	templ;
 	byte*			cached_rec;
         ut_ad(prebuilt->n_fetch_cached > 0);
+	ut_ad(prebuilt->mysql_prefix_len <= prebuilt->mysql_row_len);
 	
-	if (prebuilt->keep_other_fields_on_keyread)
+	if (UNIV_UNLIKELY(prebuilt->keep_other_fields_on_keyread))
 	{
 		/* Copy cache record field by field, don't touch fields that 
 		are not covered by current key */
@@ -2877,7 +2878,7 @@ row_sel_pop_cached_row_for_mysql(
 	else
 	{
 		ut_memcpy(buf, prebuilt->fetch_cache[prebuilt->fetch_cache_first],
-				prebuilt->mysql_row_len);
+				prebuilt->mysql_prefix_len);
 	}
 	prebuilt->n_fetch_cached--;
 	prebuilt->fetch_cache_first++;

sql/ha_innodb.cc

@@ -2618,6 +2618,8 @@ build_template(
 	ibool		fetch_all_in_key	= FALSE;
 	ibool		fetch_primary_key_cols	= FALSE;
 	ulint		i;
+	/* byte offset of the end of last requested column */
+	ulint		mysql_prefix_len	= 0;
 
 	if (prebuilt->select_lock_type == LOCK_X) {
 		/* We always retrieve the whole clustered index record if we
@@ -2738,6 +2740,11 @@ build_template(
 					get_field_offset(table, field);
 
 		templ->mysql_col_len = (ulint) field->pack_length();
+		if (mysql_prefix_len < templ->mysql_col_offset
+				+ templ->mysql_col_len) {
+			mysql_prefix_len = templ->mysql_col_offset
+				+ templ->mysql_col_len;
+		}
 		templ->type = index->table->cols[i].type.mtype;
 		templ->mysql_type = (ulint)field->type();
 
@@ -2760,6 +2767,7 @@ build_template(
 	}
 
 	prebuilt->n_template = n_requested_fields;
+	prebuilt->mysql_prefix_len = mysql_prefix_len;
 
 	if (index != clust_index && prebuilt->need_to_access_clustered) {
 		/* Change rec_field_no's to correspond to the clustered index