| Date post: | 30-May-2018 |
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8/14/2019 Adams - Hash Clusters Oracle
1/17
Steve [email protected]
Unix + Oracle =
Using Hash Clusters
8/14/2019 Adams - Hash Clusters Oracle
2/17
Steve [email protected]
Unix + Oracle =
Table Directory Row Directory
0
1
0
1
2
3
40
1
2
Cluster Blocks
Table directory
partitions the row directory
row numbers are relative to their table
cluster keys are stored as table 0
8/14/2019 Adams - Hash Clusters Oracle
3/17
Steve [email protected]
Unix + Oracle =
Cluster Key Changes
Performed by recursive/service transactions commit independent of parent/user transaction
improves concurrency
Single threaded to prevent incompatible key changes
always use ITL slot 1
slot 1 is reserved - user transactions never use it
wait in shared mode for TX lock if slot is busy
8/14/2019 Adams - Hash Clusters Oracle
4/17
Steve [email protected]
Unix + Oracle =
Index Cluster Inserts
Key can be inserted to block if number of keys < (data area size - 14) / SIZE
available space > SIZE - 27
key and data row pieces fit below PCTFREE Row inserts
lookup key in cluster index
insert to (last) data block if space permits try up to 3 previous blocks in the key chain
link a freelist block into the key chain & insert
8/14/2019 Adams - Hash Clusters Oracle
5/17
Steve [email protected]
Unix + Oracle =
Index Cluster Deletes
Data row piece headers have a key index
Keys have current & pending row counts
both incremented on insert
pending decremented on delete current decremented on stub cleanout
Flushable keys
current count was, or pending count is zero when inserting a new key, try to clean out all
committed stubs and flush keys if possible
8/14/2019 Adams - Hash Clusters Oracle
6/17
Steve [email protected]
Unix + Oracle =
Demonstration
Flushable keys
Key insert restrictions
8/14/2019 Adams - Hash Clusters Oracle
7/17
Steve [email protected]
Unix + Oracle =
Demonstration
Index clusters row inserts
8/14/2019 Adams - Hash Clusters Oracle
8/17
Steve [email protected]
Unix + Oracle =
Cluster Key Scans
Purpose to fetch all rows for a clustered table
with a known cluster key value
Buffer gets
visit all blocks in the key chain (reverse order)
scan row directory entries for the tableand check cluster key index forallrows
8/14/2019 Adams - Hash Clusters Oracle
9/17
Steve [email protected]
Unix + Oracle =
Assessing Cluster Key Chaining
Detection compare cluster key scan block gets
to the cluster key scans statistic
maybe DBA_CLUSTERS .AVG_BLOCKS_PER_KEY
Fix
rename existing tables and create new cluster use PL/SQL iterate over the cluster keys
and copy all rows for each key in turn
8/14/2019 Adams - Hash Clusters Oracle
10/17
Steve [email protected]
Unix + Oracle =
Hash Theory
TBD
8/14/2019 Adams - Hash Clusters Oracle
11/17
Steve [email protected]
Unix + Oracle =
Hash Clusters
Fixed blocks formatted during cluster creation
hash key value implied by location
keys never flushed; blocks never freed no data row counts, or previous key rowid needed
Chained blocks
just like index cluster blocks key data is hash key value
8/14/2019 Adams - Hash Clusters Oracle
12/17
Steve [email protected]
Unix + Oracle =
Demonstration
Hash cluster
8/14/2019 Adams - Hash Clusters Oracle
13/17
Steve [email protected]
Unix + Oracle =
Single Table Hash Clusters
Better single row per hash key
Reserved slots
one row slot per key
cluster key scans only inspect that slotiff no keys have more than one row
otherwise scan allrow slots for table
A flag in data layer flag byte no more than one row per key
8/14/2019 Adams - Hash Clusters Oracle
14/17
Steve [email protected]
Unix + Oracle =
Demonstration
Single table hash cluster
8/14/2019 Adams - Hash Clusters Oracle
15/17
Steve [email protected]
Unix + Oracle =
Hash Functions
Internal hash function returns 31-bit positive integer
good distribution, but not perfect
mod(hash value, HASHKEYS)
HASH IS column_name
must be integer column
saves hash computation no false collisions
8/14/2019 Adams - Hash Clusters Oracle
16/17
Steve [email protected]
Unix + Oracle =
Demonstration
Single table hash cluster without HASH IS
8/14/2019 Adams - Hash Clusters Oracle
17/17
Steve [email protected]
Unix + Oracle =
Hash Cluster Data Density
Use SINGLE TABLE syntax if possible Otherwise
2K tablespace block size
no SIZE parameter HASHKEYS is number of fixed blocks needed
primary key columns first in row
Benefits minimize CPU cost of cluster key scans
maximize data density - only one key per block
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