Value Name(s) Table method (TM lock)
0 No lock n/a
1 Null lock (NL) Used during some parallel DML operations (e.g. update) by
the pX slaves while the QC is holding an exclusive lock.
2 Sub-share (SS) Until 9.2.0.5/6 "select for update"
Row-share (RS) Since 9.2.0.1/2 used at opposite end of RI during DML
Lock table in row share mode
Lock table in share update mode
3 Sub-exclusive(SX) Update (also "select for update" from 9.2.0.5/6)
Row-exclusive(RX) Lock table in row exclusive mode
Since 11.1 used at opposite end of RI during DML
4 Share (S) Lock table in share mode
Can appear during parallel DML with id2 = 1, in the PX slave sessions
Common symptom of "foreign key locking" (missing index) problem
5 share sub exclusive (SSX) Lock table in share row exclusive mode
share row exclusive (SRX) Less common symptom of "foreign key locking" but likely to be more
frequent if the FK constraint is defined with "on delete cascade."
6 Exclusive (X) Lock table in exclusive mode
Summary of Locks Obtained by DML Statements
| SQL Statement | Row Locks | Table Lock Mode | RS | RX | S | SRX | X |
|---|---|---|---|---|---|---|---|
SELECT … FROM table... |
— | none | Y | Y | Y | Y | Y |
INSERT INTO table … |
Yes | SX | Y | Y | N | N | N |
UPDATE table … |
Yes | SX | Y* | Y* | N | N | N |
MERGE INTO table … |
Yes | SX | Y | Y | N | N | N |
DELETE FROM table … |
Yes | SX | Y* | Y* | N | N | N |
SELECT … FROM table FOR UPDATE OF … |
Yes | SX | Y* | Y* | N | N | N |
LOCK TABLE table IN … |
— | ||||||
ROW SHARE MODE |
SS | Y | Y | Y | Y | N | |
ROW EXCLUSIVE MODE |
SX | Y | Y | N | N | N | |
SHARE MODE |
S | Y | N | Y | N | N | |
SHARE ROW EXCLUSIVE MODE |
SSX | Y | N | N | N | N | |
EXCLUSIVE MODE |
X | N | N | N | N | N | |
| * Yes, if no conflicting row locks are held by another transaction. Otherwise, waits occur. | |||||||
| mode 1: | NL | Null | N | ||||
| mode 2: | SS | RS | Row-S | Row Share(d) | SubShare | Intended Share (IS) | L |
| mode 3: | SX | RX | Row-X | Row Exclusive | SubExclusive | Intended Exclusive (IX) | R |
| mode 4: | S | Share | S | ||||
| mode 5: | SSX | SRX | S/Row-X | Share(d) Row Exclusive | Share-SubExclusive | C | |
| mode 6: | X | Exclusive | X |
| compatible ? | SS,RS | SX,RX | S | SSX,SRX | X |
| SS,RS | yes | yes | yes | yes | no |
| SX,RX | yes | yes | no | no | no |
| S | yes | no | yes | no | no |
| SSX, SRX | yes | no | no | no | no |
| X | no | no | no | no | no |
GES (global enqueue resources) enqueues having different values for the lock mode:
#define KJUSERNL 0 /* no permissions */ (Null) #define KJUSERCR 1 /* concurrent read */ (Row-S (SS)) #define KJUSERCW 2 /* concurrent write */ (Row-X (SX)) #define KJUSERPR 3 /* protected read */ (Share) #define KJUSERPW 4 /* protected write */ (S/Row-X (SSX)) #define KJUSEREX 5 /* exclusive access */ (Exclusive) Global Wait-For-Graph(WFG) at ddTS[0.db] : BLOCKED 0xd876a630 5 wq 2 cvtops x1 TX 0x70015.0x81e(ext 0x2,0x0)[2B000-0001-0000057A] inst 1 BLOCKER 0xd8767a10 5 wq 1 cvtops x28 TX 0x70015.0x81e(ext 0x2,0x0)[2E000-0001-00000347] inst 1 BLOCKED 0xd876ab70 5 wq 2 cvtops x1 TX 0x40008.0x7d9(ext 0x2,0x0)[2E000-0001-00000347] inst 1 BLOCKER 0xd876a7f0 5 wq 1 cvtops x28 TX 0x40008.0x7d9(ext 0x2,0x0)[2B000-0001-0000057A] inst 1 5 means KJUSEREX ,cross instance "TX mode 6" locks
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