Re: Query optimiser is not using 'not null' constraint when 'order by nulls last' clause is used

[Nandakumar M]

Hi,

On Fri, Feb 2, 2018 at 9:28 PM, David G. Johnston
 wrote:

> You probably can (I assume the nulls aspect of the index doesn't prevent PK
> usage), but you must add the PK to the table after creating the index and
> not let the system auto-generate the index for you.
>
> https://www.postgresql.org/docs/10/static/sql-altertable.html
>
> ALTER TABLE name ADD PRIMARY KEY USING INDEX index_name;
>

I missed to notice this in the docs. Thank you David for pointing it out.

Regards,
Nanda

Re: Query optimiser is not using 'not null' constraint when 'order by nulls last' clause is used

[David G. Johnston]

On Fri, Feb 2, 2018 at 8:49 AM, Nandakumar M  wrote:

> But, for the PK column we are not in control of the index that is created.
>

​You probably can (I assume the nulls aspect of the index doesn't prevent
PK usage), but you must add the PK to the table after creating the index
and not let the system auto-generate the index for you.​

https://www.postgresql.org/docs/10/static/sql-altertable.html

​ALTER TABLE name ADD ​PRIMARY KEY USING INDEX index_name;

David J.

Re: Query optimiser is not using 'not null' constraint when 'order by nulls last' clause is used

[Nandakumar M]

Hi,

On Fri, Feb 2, 2018 at 8:30 PM, Tom Lane  wrote:
>
> The planner does not consider this and it doesn't really seem like
> something worth expending cycles on.  If you know that there won't be
> nulls in the column, why are you insisting on specifying a nondefault
> value of NULLS FIRST/LAST in the query?

The query is generated by a framework that adds 'nulls last' to all
order by clause.

This is done apparently to provide common behaviour in our application
irrespective of the database that is used.
SQL server treats nulls as lesser than non null values which is
opposite to what Postgres does.

For any indexes that we create manually, we can do a

--> create index on table_name(column_name nulls first);

But, for the PK column we are not in control of the index that is created.

Regards,
Nanda

Re: Query optimiser is not using 'not null' constraint when 'order by nulls last' clause is used

[Tom Lane]

Nandakumar M  writes:
> The order by column has a not null constraint on it and so nulls last or
> first shouldn't make any difference.

The planner does not consider this and it doesn't really seem like
something worth expending cycles on.  If you know that there won't be
nulls in the column, why are you insisting on specifying a nondefault
value of NULLS FIRST/LAST in the query?

regards, tom lane

Re: Query optimiser is not using 'not null' constraint when 'order by nulls last' clause is used

[Nandakumar M]

Hi,

On 2 Feb 2018 15:06, "Laurenz Albe"  wrote:


>In the above case, the optimizer does >not know that it will get the rows
>in the correct order: indexes are >sorted ASC NULLS LAST by default,
>so a backwards index scan will >produce the results NULLS FIRST,
>which is the default for ORDER BY ... >DESC.


The order by column has a not null constraint on it and so nulls last or
first shouldn't make any difference.


>If you want the nulls last, PostgreSQL >has to retrieve *all* the rows and
sort
>them rather than using the first 25 >results it gets by scanning then
>indexes.

>To have the above query perform >fast, add additional indexes with either
>ASC NULLS FIRST or DESC NULLS >LAST for all used keys.


For now this is exactly what I have done. But it is in effect a duplicate
index on a PK column and I would be happy not to create it in the first
place.

Regards
Nanda

Re: Query optimiser is not using 'not null' constraint when 'order by nulls last' clause is used

[Laurenz Albe]

On Thu, 2018-02-01 at 20:00 +0530, Nandakumar M wrote:
> Hi,
> 
> I am using Postgres version 9.4.4 on a Mac machine.
> I have 2 queries that differ only in the order by clause.
> One of it has 'nulls last' and the other one does not have it.
> The performance difference between the two is considerable.
> 
> The slower of the two queries is
> 
> SELECT [...]
> FROM   workorder wo
> left join  workorder_fields wof
> ON wo.workorderid=wof.workorderid
> left join  servicecatalog_fields scf
> ON wo.workorderid=scf.workorderid
[...]
> ORDER BY   7 DESC nulls last limit 25
> 
> 
> 
> On removing 'nulls last' from the order by clause the query becomes very fast.
> I have attached the query plan for both the queries.

In the above case, the optimizer does not know that it will get the rows
in the correct order: indexes are sorted ASC NULLS LAST by default,
so a backwards index scan will produce the results NULLS FIRST,
which is the default for ORDER BY ... DESC.

If you want the nulls last, PostgreSQL has to retrieve *all* the rows and sort
them rather than using the first 25 results it gets by scanning then indexes.

To have the above query perform fast, add additional indexes with either
ASC NULLS FIRST or DESC NULLS LAST for all used keys.

Yours,
Laurenz Albe

Query optimiser is not using 'not null' constraint when 'order by nulls last' clause is used

[Nandakumar M]

Hi,

I am using Postgres version 9.4.4 on a Mac machine. I have 2 queries that
differ only in the order by clause. One of it has 'nulls last' and the
other one does not have it. The performance difference between the two is
considerable.

The slower of the two queries is

SELECT wos.notificationstatus,
   wos.unrepliedcount,
   wos.shownotestotech,
   wos.ownerid,
   wos.isfcr,
   aau.user_id,
   wo.workorderid AS "WOID",
   wo.is_catalog_template AS "TemplateType",
   wo.title   AS "Title",
   wo.is_catalog_template,
   aau.first_name  AS "Requester",
   cd.categoryname AS "Category",
   ti.first_name   AS "Owner",
   wo.duebytimeAS "DueBy",
   wo.fr_duetime,
   wo.completedtime AS "CompletedTime",
   wo.respondedtime AS "RespondedTime",
   wo.resolvedtime  AS "ResolvedTime",
   qd.queuename AS "Group",
   std.statusname   AS "Status",
   wo.createdtime   AS "CreatedDate",
   wos.isread,
   wos.hasattachment,
   wos.appr_statusid,
   wos.priorityid,
   wo.templateid AS "TemplateId",
   pd.priorityid,
   pd.priorityname  AS "Priority",
   pd.prioritycolor AS "PriorityColor",
   wos.isoverdue,
   wos.is_fr_overdue,
   wos.linkedworkorderid,
   wos.editing_status,
   wos.editorid,
   wos.linkedworkorderid,
   wo.isparent,
   sduser.isvipuser,
   sduser_onbehalfof.isvipuser AS "ONBEHALFOFVIP",
   wo.isparent,
   wos.statusid,
   sdorganization.name AS "Site",
   wo.workorderid  AS "RequestID"
FROM   workorder wo
left join  workorder_fields wof
ON wo.workorderid=wof.workorderid
left join  servicecatalog_fields scf
ON wo.workorderid=scf.workorderid
left join  wotoprojects wtp
ON wo.workorderid=wtp.workorderid
left join  sitedefinition
ON wo.siteid=sitedefinition.siteid
left join  sdorganization
ON sitedefinition.siteid=sdorganization.org_id
inner join workorderstates wos
ON wo.workorderid=wos.workorderid
left join  categorydefinition cd
ON wos.categoryid=cd.categoryid
left join  aaauser ti
ON wos.ownerid=ti.user_id
left join  aaauser aau
ON wo.requesterid=aau.user_id
left join  prioritydefinition pd
ON wos.priorityid=pd.priorityid
left join  statusdefinition std
ON wos.statusid=std.statusid
left join  workorder_queue wo_queue
ON wo.workorderid=wo_queue.workorderid
left join  queuedefinition qd
ON wo_queue.queueid=qd.queueid
left join  departmentdefinition dpt
ON wo.deptid=dpt.deptid
left join  leveldefinition lvd
ON wos.levelid=lvd.levelid
left join  modedefinition mdd
ON wo.modeid=mdd.modeid
left join  urgencydefinition urgdef
ON wos.urgencyid=urgdef.urgencyid
left join  impactdefinition impdef
ON wos.impactid=impdef.impactid
left join  requesttypedefinition rtdef
ON wos.requesttypeid=rtdef.requesttypeid
left join  subcategorydefinition scd
ON wos.subcategoryid=scd.subcategoryid
left join  itemdefinition icd
ON wos.itemid=icd.itemid
left join  servicedefinition serdef
ON wo.serviceid=serdef.serviceid
left join  aaauser cbau
ON wo.createdbyid=cbau.user_id
left join  aaauser oboaau
ON wo.oboid=oboaau.user_id
left join  sduser
ON wo.requesterid=sduser.userid
left join  sduser sduser_onbehalfof
ON wo.oboid=sduser_onbehalfof.userid
left join  workorder_fields
ON wo.workorderid=workorder_fields.workorderid
WHERE  ((
 wos.statusid = 1)
   AND(
 wo.isparent = TRUE))
ORDER BY   7 DESC nulls last limit 25



On removing 'nulls last' from the order by clause the query becomes very
fast. I have attached the query plan for both the queries.

>From the plan it looks like the second query is able to efficiently use the
workorder_pk index ( The node 'Index Scan Backward using workorder_pk on
workorder' returns 25 rows) whereas the first query is not able to use the
index efficiently (more than 300k rows are returned from the same node).

The column workorderid is a PK column. The query optimizer should ideally
know that there is no nulls in this column and in effect there is no
difference between the two queries.

I tried the same in Postgres 10 and the slower query performs much better
due to parallel sequential scans but still it is less efficient than the
query without 'nulls last'.

I thought it would be best to raise this with the Postgres team.

Regards,
Nanda
pg_9_4_Fast

  QUERY PLAN