Commit
This commit does not belong to any branch on this repository, and may belong to a fork outside of the repository.
#665: [YSQL] Enable support for sequences
Summary:
This diff adds support for:
- CREATE SEQUENCE
- DROP SEQUENCE
- nextval
- currval
- lastval
- Create a table with columns of serial types
Our implementation of sequences uses one replicated user level table to store all sequences data. This table has four columns: db_oid, rel_oid, last_val, and is_called. The first two columns uniquely identify a sequence. last_val and is_called is the data necessary to determine what is the next value in the sequence. Our implementation differs from Postgres implementation by using one row in a special table as opposed to using a one-row table to store the same data.
Initially, when a sequence is created, an RPC to insert a new row is sent to the tserver that is the leader of the tablet which will store the new row. Initially last_val is set to the start value (default 1), and is_called is set to false. is_called is false when last_val hasn't been used. In other words, if nextval() reads the sequence data, it returns last_val if is_called is false. It returns last_val + increment otherwise.
Postgres implementation of nextval locks the sequence table, reads the data, checks whether incrementing (possibly by a negative value) would violate any constrains, writes the new values for last_val and is_called, and then it unlocks the table. In YugaByte's implementation we cannot lock the data table because this table is shared among all the sequences. Instead, a nextval() call sends a read RPC to read the current values `last_val` and `is_called`, constraints are then evaluated for these values, and if no errors occurred, we do a conditional update (update the row only if the values haven't changed since we last read them). If the conditional update fails, we retry the whole operation again: read data, check constraints, update data. Because our implementation uses two RPCs each time we increment `last_val` or change `is_called`, the performance of a default sequence (with `CACHE` set to 1) will be much lower than a similar sequence in Postgres. To minimize this, the user should use a cache sufficiently large to avoid issuing two RPCs for each sequence value requested through nextval(). The disadvantage of this approach, is that once a block of cache numbers has been generated, any unused numbers from the cache will be lost forever.
Pending:
- Support for CYCLE option
- ALTER SEQUENCE
- setval
Test Plan:
Manual for now. Tests coming soon:
```
postgres=# create sequence s1 increment 3 start 100 cache 1000 ;
CREATE SEQUENCE
postgres=# select nextval('s1');
nextval
---------
100
(1 row)
postgres=# select nextval('s1');
nextval
---------
103
(1 row)
postgres=# ^D\q
dog.local:~/code/yugabyte [postgres_sequence ↓·2↑·1|✚ 5⚑ 7]
14:22 $ ./bin/yb-ctl destroy; ./bin/yb-ctl create --enable_postgres; ./bin/yb-ctl status; ./bin/yb-ctl setup_pg_sequences_table; ./bin/psql -p 5433 -U postgres -h localhost^C
dog.local:~/code/yugabyte [postgres_sequence ↓·2↑·1|✚ 5⚑ 7]
14:22 $ ./bin/psql -p 5433 -U postgres -h localhost
psql (10.4)
Type "help" for help.
postgres=# select nextval('s1');
nextval
---------
3100
(1 row)
postgres=# ^D\q
dog.local:~/code/yugabyte [postgres_sequence ↓·2↑·1|✚ 5⚑ 7]
14:23 $ ./bin/psql -p 5433 -U postgres -h localhost
psql (10.4)
Type "help" for help.
postgres=# select nextval('s1');
nextval
---------
6100
(1 row)
```
```
psql (10.4)
Type "help" for help.
postgres=# create table t(k serial primary key, v int);
insert CREATE TABLE
postgres=# insert into t(v) values (100);
INSERT 0 1
postgres=# insert into t(v) values (101);
INSERT 0 1
postgres=# insert into t(v) values (102);
INSERT 0 1
postgres=# select * from t;
k | v
---+-----
1 | 100
2 | 101
3 | 102
(3 rows)
postgres=# \d t;
Table "public.t"
Column | Type | Collation | Nullable | Default
--------+---------+-----------+----------+------------------------------
k | integer | | not null | nextval('t_k_seq'::regclass)
v | integer | | |
postgres=# select nextval('t_k_seq');
nextval
---------
4
(1 row)
postgres=# select currval('t_k_seq');
currval
---------
4
(1 row)
postgres=# select lastval();
lastval
---------
4
(1 row)
```
```
postgres=# SELECT c.relname FROM pg_class c WHERE c.relkind = 'S';
relname
---------
(0 rows)
postgres=# create sequence s1;
CREATE SEQUENCE
postgres=# create table t4(k serial, v int);
CREATE TABLE
postgres=# SELECT c.relname FROM pg_class c WHERE c.relkind = 'S';
relname
----------
s1
t4_k_seq
(2 rows)
postgres=# drop sequence s1;
DROP SEQUENCE
postgres=# drop table t4;
DROP TABLE
postgres=# SELECT c.relname FROM pg_class c WHERE c.relkind = 'S';
relname
----------
t4_k_seq
(1 row)
postgres=# drop sequence t4_k_seq;
DROP SEQUENCE
postgres=# SELECT c.relname FROM pg_class c WHERE c.relkind = 'S';
relname
---------
(0 rows)
```
Reviewers: neil, karthik, mihnea, robert
Reviewed By: robert
Subscribers: kannan, bogdan, neha, yql
Differential Revision: https://phabricator.dev.yugabyte.com/D6128- Loading branch information
