revision-id: 91efcc6392cef920aa3697dc9789830ae9cdd379 (mariadb-10.1.39-37-g91efcc6392c) parent(s): c84f390df20c9fa385351d3f21a6f0f48dde1803 author: Sergei Petrunia committer: Sergei Petrunia timestamp: 2019-05-17 19:17:19 +0300 message: Better comment from Monty for code in make_join_select --- sql/sql_select.cc | 12 ++++++++++-- 1 file changed, 10 insertions(+), 2 deletions(-) diff --git a/sql/sql_select.cc b/sql/sql_select.cc index cb8c0429674..c1bec0f1c20 100644 --- a/sql/sql_select.cc +++ b/sql/sql_select.cc @@ -10027,8 +10027,16 @@ make_join_select(JOIN *join,SQL_SELECT *select,COND *cond) /* We plan to scan all rows. Check again if we should use an index. - We could have used an column from a previous table in - the index if we are using limit and this is the first table + + There are two cases: + 1) There could be an index usage the refers to a previous + table that we didn't consider before, but could be consider + now as a "last resort". For example + SELECT * from t1,t2 where t1.a between t2.a and t2.b; + 2) If the current table is the first non const table + and there is a limit it still possibly beneficial + to use the index even if the index range is big as + we can stop when we've found limit rows. (1) - Don't switch the used index if we are using semi-join LooseScan on this table. Using different index will not