PG basics -- Logical Structure Management (locking mechanism -- table lock)

pg There are two types of locks , Table level lock and row level lock .

Table lock

Table lock classification

ACCESS  SHARE	Read only tables do not modify the table to apply for the lock mode
ROW SHARE	select for update and select for share Apply for the lock mode
ROW EXCLUSIVE	DML Apply for the lock mode
SHARE UPDATE EXCLUSIVE	VACUUM、ANALYZE、CREATE INDEX CONCURRENTLY Apply for the lock mode
SHARE	create index Apply for the lock mode
SHARE ROW EXCLUSIVE	Won't apply voluntarily
EXCLUSIVE	Won't apply voluntarily
ACCESS EXCLUSIVE	alter table ,drop table,truncate,reindex ,vacuum full Apply for the lock of this mode

Table lock overview

Table lock only share and EXCLUSIVE Two kinds of , That is, read and write lock ,share The table cannot be modified after the read lock is added , Lock exclusive You cannot read or write after writing the lock , But after the multi version function , Modify a row of data , In fact, it doesn't change the original line , Instead, another row of data is copied , The modification is made on the new line , The transaction does not commit , Others cannot see the modified line , There is no need to block other users from reading data .

Under the multi version function ,ACCESS share Indicates that this lock is added , Even when the data is being modified , It is also allowed to read data , Another lock ACCESS EXCLUSIVE Even multi version features , Access to data is also not allowed .

Intent locks

The lock granularity of table lock is too large , There's a row level lock ,mysql Before obtaining row lock, you need to obtain intention lock ,pg It's similar to mysql Also have ROW_SHARE and ROW_EXCLUSIVE lock .

There is no conflict between intent locks , The conflict compatibility relationship between intentional and non intentional locks is the same as that between ordinary locks . It can be simply converted to X and S, Comparing compatibility .

SHARE UPDATE EXCLUSIVE

Is to put IX The incompatibility between locks becomes a more strict lock , For without FULL Of VACUUM,CREATE INDEX CONCURRENTLY Will apply for the lock .

SHAER ROW EXCLUSIVE

Equivalent to adding S Lock and IX lock

Table lock summary

PG The Communist Party of China 8 Seed lock , Shared lock SHARE And lock it EXCLUSIVE , Colleagues who modify data due to multiple versions are allowed to read data , Hence the ACCESS SHARE and ACCESS EXCLUSIVE lock .

In order to deal with the relationship between table lock and row lock , The concept of intention lock ,ROW_SHARE and ROW_EXCLUSIVE lock , But there will be no conflict between intention locks , To be more strict , There is SHARE

UPDATE EXCLUSIVE and SHARE ROW EXCLUSIVE lock .