An index can seek by a subset of characters, as long as you're searching from the left. E.g., "Inter%" can seek, "%net" will not.
However, the first character is not necessarily the character under which the article would be sorted. "The Internet" should go under "I", not "T". You probably need two fields, DisplayTitle
and SortTitle
; a single-character index on the latter may be worthwhile, but most likely a full-length index will be just fine.
Indexes are typically B-trees, and a seek will jump to the right location about equally quickly whether you have 10 or 100 entries per page. Scans are another matter, but I'd start with the simplest solution and add an extra index only if performance proves inadequate in practice.
Your indexes are fine for the two types of queries you mentioned.
This query will be satisfied by traversing the clustered index on the primary key...
[...] WHERE participant_id = x AND question_id = y AND given_answer_id = z;
...and this one is satisfied by the index on 'question_id':
[...] WHERE question_id = x;
The output of EXPLAIN SELECT
is not telling you what you think it is telling you, because the value shown in rows
is an estimate of the number of rows the server will need to consider, not the actual rows it will examine. For InnoDB
these are based on index statistics.
rows
The rows column indicates the number of rows MySQL believes it must examine to execute the query.
For InnoDB tables, this number is an estimate, and may not always be exact.
— http://dev.mysql.com/doc/refman/5.5/en/explain-output.html#explain_rows
The optimizer gathers information about different possible query plans, and chooses the one with the lowest cost. The information shown in EXPLAIN
is the information the optimizer gathered about the plan it selected.
When type
is ref
and key
is not NULL
, this means that the name listed in the key
column is the name of the index that the optimizer has chosen to use to find the desired rows, so your query plan looks exactly as it should.
Note, sometimes you will see Using index
in the Extra
column and a lot of people assume that this means an index is being used, or that no index is being used when that doesn't appear, but that's not correct, either. Using index
describes a special case called a "covering index" -- it does not indicate whether an index is being used to locate the rows of interest.
It's possible that running ANALYZE [LOCAL] TABLE
would cause the numbers in rows
shown by EXPLAIN
to differ, but this is a simple query and selecting this index is an obvious choice for the optimizer to make, so ANALYZE TABLE
is unlikely to make any actual difference in performance.
It is possible, however, that your overall performance might see some marginal improvement with an occasional OPTIMIZE [LOCAL] TABLE
, because you are not inserting rows in primary key order (as would be the case with an auto_increment
primary key)... but on large tables this can be time-consuming because it rebuilds a new copy of the table... but, again, I wouldn't expect any significant change.
Best Answer
Every InnoDB table should be given an explicit
PRIMARY KEY
. It must be "unique" and can either be a column (or combination of columns) or can be anAUTO_INCREMENT
surrogate.GEN_CLUSTER_INDEX
is the fallback when you don't provide a PK.VALUES(1, 'prod1')
-- You are locking the one row from two different approaches (the secondary indexes). Hence, it is quite proper to have a lock. This lock will either lead to delaying one of the transactions until the other finished, or lead to a deadlock which will abort (and rollback) one of the transactions.I don't believe you getting a "table lock", but instead a "row lock".
If your table is a "many-to-many" mapping table, please follow the advice in http://mysql.rjweb.org/doc.php/index_cookbook_mysql#many_to_many_mapping_table