Обсуждение: Reusing Dead Tuples:

Hi,  I am doing some experiments on dead tuples,  I am looking of reusing the
dead tuples  apace in a particular page during the "Update".This patch 
is meant for the tables
which are heavily updated to avoid vacuum very frequently.By  using it 
will arrest the size of
table for heavily updated table. The algorithm works like this:1) During the update it check for the dead tuples in the
current
 
page(page that containthe tuple that need to be updated). If it finds any dead tuples it uses 
the dead tuple spaceby ovewriting on dead tuple. The checking of dead tuple is very similer 
to the task that of
lazy vaccum.2) If it cannot find any dead tuple  it proceed as usual by inserting 
at the end of table .
Performance Effect: 1) The CPU processing will be slighly more for the update, but io 
processing is
exactly same2)  The size of table grows slower under heavy update , so vacuum is 
not required very frequently. The total processing for update is more or less same   even  after 
doing large number of updates without vacuum.

Does it breaks anythings  by overwriting the dead tuples ?.

Comments?.

jana

Janardhan <jana-reddy@mediaring.com.sg> writes:
> Does it breaks anythings  by overwriting the dead tuples ?.

Yes.  You cannot do that unless you've first removed index entries
pointing at the dead tuples --- and jumped through the same locking
hoops that lazy vacuum does while removing index entries.
        regards, tom lane

Tom Lane wrote:<br /><blockquote cite="mid8875.1039581335@sss.pgh.pa.us" type="cite"><pre wrap="">Janardhan <a
class="moz-txt-link-rfc2396E"href="mailto:jana-reddy@mediaring.com.sg"><jana-reddy@mediaring.com.sg></a> writes:
</pre><blockquotetype="cite"><pre wrap="">Does it breaks anythings  by overwriting the dead tuples ?.
</pre></blockquote><prewrap="">
 
Yes.  You cannot do that unless you've first removed index entries
pointing at the dead tuples --- and jumped through the same locking
hoops that lazy vacuum does while removing index entries.
        regards, tom lane
 </pre></blockquote> if i am not wrong while  updating a tuple, we are also creating a  new index entry . so  if the<br
/>tuple is dead then the index entry pointing it also a dead index tuple. so even if dead index tuple is not <br />
removedthen also it should not break thing, since the dead index tuple will not be used, am i correct?.<br /><br />
whatis reason why the dead heap tuples are maintained in a linked list ?. since for every dead heap tuple there<br />
isa corresponding dead index tuple.<br /><br /> Regards<br /> jana<br /> 

Janardhan <jana-reddy@mediaring.com.sg> writes:
> if i am not wrong while  updating a tuple, we are also creating a  new 
> index entry .

Yes.

> so  if the
> tuple is dead then the index entry pointing it also a dead index tuple. 

Yes.

> so even if dead index tuple is not
> removed then also it should not break thing, since the dead index tuple 
> will not be used, am i correct?.

No.  A process running an indexscan will assume that the index tuple
accurately describes the heap tuple it is pointing at.  If the heap
tuple is live then it will be returned as satisfying the indexscan.
        regards, tom lane

Tom Lane wrote:<br /><blockquote cite="mid8875.1039581335@sss.pgh.pa.us" type="cite"><pre wrap="">Janardhan <a
class="moz-txt-link-rfc2396E"href="mailto:jana-reddy@mediaring.com.sg"><jana-reddy@mediaring.com.sg></a> writes:
</pre><blockquotetype="cite"><pre wrap="">Does it breaks anythings  by overwriting the dead tuples ?.
</pre></blockquote><prewrap="">
 
Yes.  You cannot do that unless you've first removed index entries
pointing at the dead tuples --- and jumped through the same locking
hoops that lazy vacuum does while removing index entries.
        regards, tom lane
 </pre></blockquote> Does it breaks any other things if all the index entries pointing the dead tuple are  removed
beforereusing the dead tuple?.<br /><br /> Regards<br /> jana<br /> 

Janardhan <jana-reddy@mediaring.com.sg> writes:
> Does it breaks any other things if all the index entries pointing the 
> dead tuple are  removed before reusing the dead tuple?.

Possibly you could make that work, but I think you'll find the
efficiency advantage you were chasing to be totally gone.  The locking
scheme is heavily biased against you, and the index AMs don't offer an
API designed for efficient retail index-tuple deletion.

Of course that just says that you're swimming against the tide of
previous optimization efforts.  But the thing you need to face up to
is you are taking what had been background maintenance tasks (viz,
VACUUM) and moving them into the foreground critical path.  This *will*
slow down your foreground applications.
        regards, tom lane

Tom Lane wrote:<br /><blockquote cite="mid5876.1039765041@sss.pgh.pa.us" type="cite"><pre wrap="">Janardhan <a
class="moz-txt-link-rfc2396E"href="mailto:jana-reddy@mediaring.com.sg"><jana-reddy@mediaring.com.sg></a> writes:
</pre><blockquotetype="cite"><pre wrap="">Does it breaks any other things if all the index entries pointing the 
 
dead tuple are  removed before reusing the dead tuple?.   </pre></blockquote><pre wrap="">
Possibly you could make that work, but I think you'll find the
efficiency advantage you were chasing to be totally gone.  The locking
scheme is heavily biased against you, and the index AMs don't offer an
API designed for efficient retail index-tuple deletion.

Of course that just says that you're swimming against the tide of
previous optimization efforts.  But the thing you need to face up to
is you are taking what had been background maintenance tasks (viz,
VACUUM) and moving them into the foreground critical path.  This *will*
slow down your foreground applications.
        regards, tom lane
 </pre></blockquote> today i could able to complete the patch and  it is working only for b-tree.  i have added  a new
 methodam_delete <br /> to the  API and bt_delete to the B-tree index to delete a single entry.   for the timebeing
thisworks only with<br /> b-tree indexs.<br /><br /> Regarding the complexity of deleting a  tuple from b-tree , it is
sameor less then that of<br /> inserting a tuple into a B-tree( since delete does not require spliting the page). The
approachis  slightly  <br /> different to that of lazy vacuum. lazy vacuum scan entire index table to delete the dead
entries.<br/> here it search for the pariticilar entry similer to that of insert .  <br /> here locking may not have
muchimpact. It locks only  single buffer to delete  the index entry.<br /><br /> Regarding the efficiency, if the
entireIndex table is in buffered then it does not require any <br /> additional IO , only extra CPU is required to
deleteentries in index table.<br /> I am using postgres in a application where is there is heavy updates for group of
tables(smallsize), before inserting<br /> a single record in huge table. this entire thing constitue single
transaction.currently as time goes on the transaction <br /> processing speed decreases till the database is vacuumed.
<br/><br /> Using this new patch i am hoping the trasaction processing time will remain constant irrespective of time.
Onlyi need to do<br /> vaccum once i delete large number of entries from some of the tables.<br /><br /> regards,
jana<br/><br />