Does inserting/ updating or deleting a record in DB auto update the indexes?
I've seen rebuild commands for indexes and I just can't understand if I have to do all these manually, then what's the purpose of indexes. This is so inconvenient that I spend time rebuilding the indexes every week or so.
When index fragmentation becomes too high, indexes will be very inefficient. There is need to rebuild index.
Let SQL Server Tell You Which Indexes to Rebuild
Rebuilding an index drops and re-creates the index. This removes fragmentation, reclaims disk space by compacting the pages based on the existing fill factor setting, and reorders the index rows in contiguous pages.
Reorganizing and Rebuilding Indexes
Yes, when you insert, update, or delete a row, any indexes on that table automatically reflect the change.
Related
The issue I'm trying to solve is index rebuild that keeps consuming the entire disc space (and then fails the operation).
The command in use is (while iterating over all tables in the DB):
ALTER INDEX ALL ON MyTable REBUILD
SQL Server 2014
Total disk space is 465 GB
The database size is ~320GB
During index rebuild the disc is filled entirely. And when the operation fails, the space is not deallocated. That means no free space is left on the disk - at all.
Any suggestions regarding what can be done differently?
Thank you.
Any suggestions regarding what can be done differently?
Don't rebuild all your indexes. Why are you even doing that? Try just updating statistics instead.
Rebuild the indexes one-by one.
If a single index is too large to rebuild, consider disabling it first, which is like dropping the index but retaining the index definition. Warning, though, disabling the primary key index (or a unique index referenced by a foreign key) or a clustered index has side effects. Foreign keys on other tables will be disabled (NOCKECK), and if the clustered index is disabled all other indexes on the table will be disabled too.
And when the operation fails, the space is not deallocated.
When a database is set to auto-grow, it grows. There is no "auto shrink" setting for a database. To reclaim space allocated to a database after a on-time operation you can shrink it.
And if you absolutely have to rebuild all indexes for some reason, then get enough drive space to do so with room to spare.
Do not shrink constantly (e.g., as part of a maintenance plan).
It's OK to shrink in rare/emergency situations when you have no option, but ironically, can fragment the very indexes your were trying to rebuild.
I need to know that if we delete some rows (I am talking for sql server) from a table which has some indexes (clustered or non-clustered, for both situation) can give any damage to indexes or not? What happens to indexes when we delete rows? Which one is better for performance, deleting rows from a table after processing them, or mark up them as processed (When we will need to reuse them like 20 times more). Thanks for the answers.
I don't know what you mean by "damage". When you delete rows from the table, the index entries need to be deleted as well. This does not "damage" the index per se. At least, the index continues to be useful.
If you have lots of deletes, updates, and inserts, then over time the index will be fragmented. This does affect performance. At some point it becomes useful to re-build the index for performance purposes. You can read about this in the documentation.
I would not worry about rebuilding the indexes because of a handful of deletes. It takes a bit of work to really fragment an index.
My answer is YES.
Index is created on data in the tables and in short if data is deleted from the tables then the levels of fragmentation rise.
Rise in fragmentation levels effects the data retrieval in many ways.
We have a range partitioned table and about 10 bitmap local indexes for that table. We perform some ddl/dml operations on that table in our daily load, which is truncate a specific partition and load data. when we do this, the local bitmap indexes are not becoming unusable. They are in usable status. However, my question is, even though the indexes are not getting unusable, do we always need to incorporate index rebuilding as part of the best practice for range partitioned tables, or use the index rebuilding only when it is required? because index rebuilding takes time, imagine we have 10 local indexes on that table which has large volume, then it becomes a costly affair for etl.
Please provide me your suggestions or thoughts in this situation?
No a rebuild of local indexes is not required, that is one of the main purpose of an local index.
Local partitioned indexes actually create 'sub index' for each partition, so such 'sub index' can be managed independently from other partitions. And when you truncate partition all its local indexes are truncated either.
Oracle doc:
"You cannot truncate an index partition. However, if local indexes are
defined for the table, the ALTER TABLE ... TRUNCATE PARTITION
statement truncates the matching partition in each local index."
So when you load data to that partition it recreate local indexes. But statistic on that index could be wrong and optimizer can consider don't use index. So consider gathering statistics from such indexes if you don't do it.
I removed some rows from a very large table. Then I ran a query that usually runs within a few seconds and found it to be running very slowly after deleting the rows. I re-built my index and ran my query and found it to be fast again. Could deleting those rows caused the index to be fragmented?
Yes, deleting rows affects the index and maintenance should take place to keep the index relatively in sync with existing data.
Rebuilding an index was likely unnecessary - you only need to do this if the physical fragmentation is 30 percent or more according to MS documentation. REORGANIZE is usually a better choice - think of it as defraging the index.
This is a good article series on SQL Server Index Fragmentation.
What is the difference between Index Rebuilding and Index Reorganizing?
Think about how the index is implemented. It's generally some kind of tree, like a B+ Tree or B- Tree. The index itself is created by looking at the keys in the data, and building the tree so the table can be searched efficiently.
When you reorganize the index, you go through the existing index, cleaning up blocks for deleted records etc. This could be done (and is in some databases) when you make a deletion, but that imposes some performance penalty. instead, you do it separately in order to do it more or less batch mode.
When you rebuild the index, you delete the existing tree and read all the records, building a new tree directly from the data. That gives you a new, and hopefully optimized, tree that may be better than the results of reorganizing the table; it also lets you regenerate the tree if it somehow has been corrupted.
REBUILD locks the table for the whole operation period (which may be hours and days if the table is large).
REORGANIZE doesn't lock the table.
Well. actually, it places some temporary locks on the pages it works with right now, but they are removed as soon as the operation is complete (which is fractions of second for any given lock).
As #Andomar noted, there is an option to REBUILD an index online, which creates the new index, and when the operation is complete, just replaces the old index with the new one.
This of course means you should have enough space to keep both the old and the new copy of the index.
REBUILD is also a DML operation which changes the system tables, affects statistics, enables disabled indexes etc.
REORGANIZE is a pure cleanup operation which leaves all system state as is.
There are a number of differences. Basically, rebuilding is a total rebuild of an index - it will build a new index, then drop the existing one, whereas reorganising it will simply, well... it will reorganise it.
This blog entry I came across a while back will explain it much better than I can. :)
Rebuild it droping the current indexes and recreating new ones.
Reorganizing is like putting the house in order with what u already have.
it is a good practice to use 30% fragmentation to determine rebuild vs reorganize.
<30% reorganize vs. >30% rebuild
"Reorganize index" is a process of cleaning, organizing, and defragmenting of "leaf level" of the B-tree (really, data pages).
Rebuilding of the index is changing the whole B-tree, recreating the index.
It’s recommended that index should be reorganized when index fragmentation is from 10% to 40%; if index fragmentation is great than 40%, it’s better to rebuild it.
Rebuilding of an index takes more resources, produce locks and slowing performance (if you choose to keep table online). So, you need to find right time for that process.
In addition to the differences above (basically rebuild will create the index anew, and then "swap it in" for the existing one, rather than trying to fix the existing one), an important consideration is that a rebuild - even an Enterprise ONLINE rebuild - will interfere with snapshot isolation transactions.
TX1 starts snapshot transaction
TX1 reads from T
TX2 rebuilds index on T
TX2 rebuild complete
TX1 read from T again:
Error 3961, Snapshot isolation transaction failed in database because the object accessed by the statement has been modified by a DDL statement in another concurrent transaction since the start of this transaction. It is disallowed because the metadata is not versioned. A concurrent update to metadata can lead to inconsistency if mixed with snapshot isolation.
Rebuild index - rebuilds one or more indexes for a table in the specified database.
Reorganise index - Defragments clustered and secondary indexes of the specified table