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IDS Forum

Re: create indexes before or after inserting rows?

Posted By: Art Kagel
Date: Friday, 29 May 2015, at 4:29 p.m.

In Response To: create indexes before or after inserting rows? (BEN KOPACZ)


Great set of questions and I have lots of answers. Here goes, last
questions first:

- Does my proposed .... What role does updating the statistics play?

- You don't say what version you are running which is always a good idea

since sometimes answers have to be version specific. In
versions 11.70 and

later Informix produces HIGH statistics for the leading column
in the index

as part of the index build. If the index is empty, those distributions

will be empty. Of course, since you intend to run the update statistics

after loading the data, this is not a big issue assuming that you run the

recommended suite of update statistics commands from the
Performance Guide

(or use my dostats utility).

- By creating the indexes after loading the data, the load will go

faster and most of the time the savings will be more than the time to build

the index on the full table assuming you can build the index using

PDQPRIORITY or parallel sorting (PSORT_NPROCS). That is why Server Studio

uses this method.

- Your method as layed out will work fine. Make sure that the ALTER

TABLE EXTENT SIZE is run before the TRUNCATE TABLE and that you do not

include the REUSE STORAGE option or the initial extent's size will not be


- There is an alternative method to reorg the table and compress out the

extents. It usually results in one or two extents in the resulting table

but has the advantage that it can be performed online without any

downtime. There will be some minor performance impact while the reorg is

running, but otherwise it is low impact. It does like this:

- Set the new first extent and next extent sizes.

- Execute the DEFRAGMENT SQL API function for the target table:

execute function sysadmin:task( 'DEFRAGMENT', 'database:tablename' );

- or -

execute function sysadmin:task('DEFRAGMENT PARTITION',

'partnum1,partnum2, ...' );

- Monitor the execution (it runs in the background) using onmode -g


- Another option is to use alter fragment to reorg the table, that will

lock the table but it is often faster than the unload - reload method:

- Set the new first extent and next extent sizes.

- ALTER FRAGMENT ON mytable INIT IN dbspace;

The dbspace can be the same dbspace that the table already resides in

or a different one.

Since neither of the methods I offer instead of the one you are planning to
use requires any indexes to be rebuilt nor update statistics to be run,
they both will be faster than your method (or using Server Studio). The
ALTER FRAGMENT method actually has to patch up the rowids in the indexes
while the DEFRAGMENT API does not. The API function leaves the relative
rowids for the rows unchanged so this method does not recover or compress
out empty row slots left by deleted rows. You can use the REPACK or REPACK
SHRINK API function to compress out deleted row space either before or
after defragmenting. Both you method and the ALTER FRAGMENT will compress
out deleted row slots.


Art S. Kagel, President and Principal Consultant
ASK Database Management


Disclaimer: Please keep in mind that my own opinions are my own opinions
and do not reflect on the IIUG, nor any other organization with which I am
associated either explicitly, implicitly, or by inference. Neither do
those opinions reflect those of other individuals affiliated with any
entity with which I am affiliated nor those of the entities themselves.

On Fri, May 29, 2015 at 3:15 PM, BEN KOPACZ <> wrote:

> Hello all,
> I'm somewhat of a newby with Informix. Im trying to figure out the point
> of
> building indexes after inserting a bunch of rows as opposed to creating
> them
> on an empty table. Creating indexes on tables with no rows takes an
> instant,
> and creating an index on a table with a lot of rows takes a significant
> amount
> of time.
> We have some tables with excessive extents I'm trying to optimize, and
> want to
> make the process fairly automated, and use Server Studio as little as
> possible. This is what I intend to do:
> - Calculate optimum extent size with server studio.
> - Unload table.
> - Make a backup of the schema.
> - Truncate the table.
> - Modify first and next extent size to optimum.
> - Reload table.
> - Update statistics.
> When Server Studio reorganizes a table, it drops the table entirely,
> recreates
> the schema without the indexes, loads the data, then builds the indexes.
> The
> index building takes the majority of the time for the entire process.
> Ive tested my mostly command line process on some test tables, and they
> show
> the indexes. Does my proposed method leave the table with inaccurate
> indexes,
> since it doesnt rebuild indexes on the reloaded data? What role does
> updating
> the statistics play?
> *******************************************************************************
> Forum Note: Use "Reply" to post a response in the discussion forum.


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