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Type:
Bug
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Resolution: Done
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Priority:
Major - P3
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Affects Version/s: WT2.6.1
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Component/s: None
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None
Hi!
We have a table with index and sometimes want to read only index.
I.e. we're only interested in pairs (index key, primary key)
So we tried to use index cursor projection as described here
But it seems that WiredTiger nevertheless reads the main table although it doesn't need to (I think).
The problem is that our main table is quite big (several GB's) and scanning through index lead to it being read non-sequentially which is quite slow.
Small demo example attached (should be build with -rdynamic)
Output I get:
$ ./read_index Filling DB... total 481716 -rw-r--r-- 1 sbn eng 757760 Aug 7 15:03 main_index.wti -rw-r--r-- 1 sbn eng 491974656 Aug 7 15:03 main.wt -rw-r--r-- 1 sbn eng 45 Aug 7 15:03 WiredTiger -rw-r--r-- 1 sbn eng 298 Aug 7 15:03 WiredTiger.basecfg -rw-r--r-- 1 sbn eng 21 Aug 7 15:03 WiredTiger.lock -rw-r--r-- 1 sbn eng 938 Aug 7 15:03 WiredTiger.turtle -rw-r--r-- 1 sbn eng 36864 Aug 7 15:03 WiredTiger.wt Reading index using index: cursor... total 0 lrwx------ 1 sbn eng 64 Aug 7 15:03 0 -> /dev/pts/2 lrwx------ 1 sbn eng 64 Aug 7 15:03 1 -> /dev/pts/2 lrwx------ 1 sbn eng 64 Aug 7 15:03 2 -> /dev/pts/2 lrwx------ 1 sbn eng 64 Aug 7 15:03 3 -> /home/sbn/src/wiredtiger/examples/c/read_index.db/WiredTiger.lock lrwx------ 1 sbn eng 64 Aug 7 15:03 4 -> /home/sbn/src/wiredtiger/examples/c/read_index.db/WiredTiger.wt lrwx------ 1 sbn eng 64 Aug 7 15:03 5 -> /home/sbn/src/wiredtiger/examples/c/read_index.db/main_index.wti lrwx------ 1 sbn eng 64 Aug 7 15:03 6 -> /home/sbn/src/wiredtiger/examples/c/read_index.db/main.wt key_format = i, value_format = i pread() fd stats: 5: 745472 6: 491962368 Reading index using file: cursor... total 0 lrwx------ 1 sbn eng 64 Aug 7 15:03 0 -> /dev/pts/2 lrwx------ 1 sbn eng 64 Aug 7 15:03 1 -> /dev/pts/2 lrwx------ 1 sbn eng 64 Aug 7 15:03 2 -> /dev/pts/2 lrwx------ 1 sbn eng 64 Aug 7 15:03 3 -> /home/sbn/src/wiredtiger/examples/c/read_index.db/WiredTiger.lock lrwx------ 1 sbn eng 64 Aug 7 15:03 4 -> /home/sbn/src/wiredtiger/examples/c/read_index.db/WiredTiger.wt lrwx------ 1 sbn eng 64 Aug 7 15:03 5 -> /home/sbn/src/wiredtiger/examples/c/read_index.db/main_index.wti key_format = ii, value_format = pread() fd stats: 5: 745472
Thanks!
- is depended on by
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SERVER-20060 WiredTiger changes for MongoDB 3.1.8
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- Closed
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