Derby documentation specifies that SELECT statements do not place any locks
in the READ_UNCOMMITTED isolation level. However, the query plan shows that
it uses shared locks (which is also confirmed by application behaviour). So
is the documentation wrong or is this a bug?
This is derby 10.2.2.0, see the query plan below:
2008-05-07 14:21:39.990 GMT Thread[ExportWorker 2,5,ArchiveGroup] (XID =
583431), (SESSIONID = 17), select ID, AGENT_SESSION_ID, SEQUENCE_NO,
SCRIPT_SESSION_ID, EVENT_TIME, EVENT_TIME_SRV, L1, L2, L3, L4, L5, TOPIC_ID,
OBJECT_ID, TEXT, ATTRIBUTES from TRACK_EVENT where AGENT_SESSION_ID in
(43909,43914,43921,43922,43923,43924,43925,43926,43927,43930,43931,43932,43933,43934,43935,43936,43937,43938,43939,43940)
******* Project-Restrict ResultSet (3):
Number of opens = 1
Rows seen = 63645
Rows filtered = 0
restriction = true
projection = true
constructor time (milliseconds) = 0
open time (milliseconds) = 0
next time (milliseconds) = 0
close time (milliseconds) = 0
restriction time (milliseconds) = 0
projection time (milliseconds) = 0
optimizer estimated row count: 16824.30
optimizer estimated cost: 127904.69
Source result set:
Index Row to Base Row ResultSet for TRACK_EVENT:
Number of opens = 1
Rows seen = 63645
Columns accessed from heap = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,
12, 13, 14}
constructor time (milliseconds) = 0
open time (milliseconds) = 0
next time (milliseconds) = 0
close time (milliseconds) = 0
optimizer estimated row count: 16824.30
optimizer estimated cost: 127904.69
Index Scan ResultSet for TRACK_EVENT using index TE_IDX_SESSION
at read uncommitted isolation level using share row locking chosen by the
optimizer
Number of opens = 1
Rows seen = 63645
Rows filtered = 0
Fetch Size = 16
constructor time (milliseconds) = 0
open time (milliseconds) = 0
next time (milliseconds) = 0
close time (milliseconds) = 0
next time in milliseconds/row = 0
scan information:
Bit set of columns fetched={0, 2}
Number of columns fetched=2
Number of deleted rows visited=0
Number of pages visited=512
Number of rows qualified=63645
Number of rows visited=63646
Scan type=btree
Tree height=4
start position:
>= on first 1 column(s).
Ordered null semantics on the following columns:
0
stop position:
> on first 1 column(s).
Ordered null semantics on the following columns:
0
qualifiers:
None
optimizer estimated row count: 16824.30
optimizer estimated cost: 127904.69
--
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