Hi, Nikita,
On Jan 09, Nikita Malyavin wrote:
>
> I don't understand the notion of the "autoincrement index". Why any index
> is important at all for an autoincrement field?
Three examples:
#####################
create table t1 (a int, b int not null auto_increment, unique ub (b));
insert t1 (a) values (1),(2),(1),(3);
select * from t1;
prints:
+------+---+
| a | b |
+------+---+
| 1 | 1 |
| 2 | 2 |
| 1 | 3 |
| 3 | 4 |
+------+---+
#####################
create table t1 (a int, b int not null auto_increment, unique uab (a,b));
insert t1 (a) values (1),(2),(1),(3);
select * from t1;
prints:
+------+---+
| a | b |
+------+---+
| 1 | 1 |
| 1 | 2 |
| 2 | 1 |
| 3 | 1 |
+------+---+
#####################
create table t1 (a int, b int not null auto_increment, unique uab (a,b),
unique ub (b));
insert t1 (a) values (1),(2),(1),(3);
select * from t1;
prints:
+------+---+
| a | b |
+------+---+
| 1 | 1 |
| 1 | 3 |
| 2 | 2 |
| 3 | 4 |
+------+---+
in the third example the auto-inc column takes part in two indexes.
But only one of them - ub - defines how auto-inc values are generated.
The "value generator" is a pair {b, ub} - a column and an index. Looking
at the column declaration alone you cannot predict how values will be
generated.
Regards,
Sergei
Chief Architect, MariaDB Server
and secur...@mariadb.org
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