Query long "statistics" step solved by using temp table, why? - mysql

Question

I have this query (4.8 seconds run time):
SELECT a, b, c
FROM table_a ta
INNER JOIN table_b tb ON ta.id = tb.id
AND ta.id2 = tb.id2
WHERE ta.id2 = 1
AND tb.id2 = 1
AND ta.id IN (*100K strings list*)
(I know the condition on id2 = 1 can be done better, lets ignore that for now)
So for the above query, when profiling, I get:
| statistics | 3.471655 |
Reading online a bit, I saw it meant the thread is performing "disk-bound other work".
After changing the query to insert the 100K strings into a temp table and joining with that table, I managed to reduce the run time to 0.82
seconds, but I cant say I completely understand why.
So:
What does "disk bound other-work" mean exactly? What determines how long this step will run? Table size row-wise? Table size bytes-wise?
Where does that run time improvement comes from? Is JOIN really that much more efficient than IN? I always figured they just create some hash set in memory and use that, which should be very fast
EDIT:
Im using MariaDB 10.2.25.
CREATE TABLE:
CREATE TABLE table_a (
d VARCHAR(100) DEFAULT NULL,
e VARCHAR(100) DEFAULT NULL,
f VARCHAR(100) DEFAULT NULL,
g VARCHAR(100) DEFAULT NULL,
h VARCHAR(100) DEFAULT NULL,
i VARCHAR(100) DEFAULT NULL,
a CHAR(32) NOT NULL,
j CHAR(32),
k CHAR(27) NOT NULL,
id BIGINT(20),
l VARCHAR(100) DEFAULT NULL,
m VARCHAR(100) DEFAULT NULL,
n VARCHAR(100) DEFAULT NULL,
o TEXT,
b INT DEFAULT NULL,
p VARCHAR(100) DEFAULT NULL,
q INT(10) DEFAULT NULL,
r INT(10) DEFAULT NULL,
s INT(10) DEFAULT NULL,
t CHAR(5) DEFAULT NULL,
u INT(3) DEFAULT NULL,
v BOOL,
w BOOL,
x BOOL,
y VARCHAR(100) DEFAULT NULL,
z VARCHAR(100) DEFAULT NULL,
dd VARCHAR(100) DEFAULT NULL,
ee VARCHAR(100) DEFAULT NULL,
ff VARCHAR(100) DEFAULT NULL,
gg VARCHAR(500) DEFAULT NULL,
hh VARCHAR(50) DEFAULT NULL,
ii VARCHAR(50) DEFAULT NULL,
jj BOOL DEFAULT NULL,
kk VARCHAR(500) DEFAULT NULL,
id2 INT NOT NULL,
ll INT UNSIGNED DEFAULT NULL,
KEY idx1 (m),
KEY idx2 (id2,id),
KEY idx3 (id2),
PRIMARY KEY (id2,a)
) DEFAULT CHARSET=utf8;
CREATE TABLE table_b (
aaa CHAR(27) NOT NULL,
id BIGINT(20),
bbb INT UNSIGNED,
c INT UNSIGNED NOT NULL,
ccc VARCHAR(50),
id2 INT UNSIGNED NOT NULL,
KEY idx1 (id2,id),
PRIMARY KEY (id2,aaa),
KEY `id` (`id`)
);
EXPLAIN
Query 1:
+------+-------------+-------+-------+----------------------------------+-------------+---------+---------------------------+--------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+------+-------------+-------+-------+----------------------------------+-------------+---------+---------------------------+--------+-------------+
| 1 | SIMPLE | table_a | range | PRIMARY,idx3,idx2 | PRIMARY | 100 | NULL | 100000 | Using where |
| 1 | SIMPLE | table_b | ref | PRIMARY,id | id | 13 | table_a.id,const | 1 | |
+------+-------------+-------+-------+----------------------------------+-------------+---------+---------------------------+--------+-------------+
EXPLAIN Query 2:
+------+-------------+-------+--------+----------------------------------+-------------+---------+---------------------------+-------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+------+-------------+-------+--------+----------------------------------+-------------+---------+---------------------------+-------+-------------+
| 1 | SIMPLE | ttt | index | PRIMARY | PRIMARY | 62 | NULL | 76191 | Using index |
| 1 | SIMPLE | table_a | eq_ref | PRIMARY,idx3,idx2 | PRIMARY | 100 | const,ttt.id | 1 | Using where |
| 1 | SIMPLE | table_b | ref | PRIMARY,id | id | 13 | table_a.id,const | 1 | |
+------+-------------+-------+--------+----------------------------------+-------------+---------+---------------------------+-------+-------------+

Answer 1

(Too much for a Comment; may lead to an Answer.)
CHAR(32) -- are these UUIDs? If so, declare them CHARACTER SET ascii, not utf8.
Be consistent on SIGNED vs UNSIGNED, especially with columns used in JOIN.
id | 13 | table_a.id,const does not agree with the table definition given. Please fix.
How much time does it take to build ttt? If that is too large, you are not saving any time.