Active Session History: Updating the X$ASH, part 03

Part 01
Part 02

Here I try to show you what happens and why Oracle updates the V$ASH view. Please read the pdf by Graham Wood, for more details.

What I analyze is the session "session_id=1732" with "session_serial#=31065" saw in the previous post.

This is what happens

Click for enlarge

The session 1732 (serial# 31065) is sampled 3 times: at sample_id 50243482, 50243483, 50243484. In the last sample_id, there is a change of state of the session. In fact, querying the V$ACTIVE_SESSION_HISTORY, you see

Click for enlarge

But what happens behind the lines is something like this. Pay attention that my conclusions are based on my experiment

Click for enlarge

(1) SAMPLE_ID 50243482/SAMPLE_TIME 15-DEC-17 02:50:40.054

At the 15/12/2017 14:50:40.838429, one row is inserted in X$ASH with SAMPLE_TIME, 14:50:40.054

(2) SAMPLE_ID 50243484/SAMPLE_TIME 15-DEC-17 02:50:42.064

At the 15/12/2017 14:50:42.016313, one row is inserted in X$ASH with SAMPLE_TIME, 14:50:41.064

(3) SAMPLE_ID 50243484/SAMPLE_TIME 15-DEC-17 02:50:41.064

At the 15/12/2017 14:50:43.192604, one row is inserted in X$ASH with SAMPLE_TIME, 14:50:42.064

Note that, at this time, the session 1732 is waiting, but Oracle doesn't know yet how match time the session will wait

(4) SAMPLE_ID 50243484/SAMPLE_TIME 15-DEC-17 02:50:41.064 (the same of the previous one)

At the 15/12/2017 14:50:44.372309 (sample_id 50243485), the last row is updated in X$ASH with SAMPLE_TIME, 14:50:42.064

At sample_id 50243485, the session 1732 finish it works and Oracle know how much time the session waited. So he can update the state of the wait on X$ASH

Here I left the previous row, just to show what happening.

(5) The final result

What you see finally, is a consistent state of the session

Part 01
Part 02

Active Session History: Updating the X$ASH, part 02

Part01
Part03

Starting from previous output, running the following query

set lines 160
col SAMPLE_TIME for a30
set pages 99
select
    sample_id
  , session_id
  , session_serial#
  , time_waited
  , count(*)
  , sample_time 
from 
    my_active_session_history 
group by 
    sample_id
  , session_id
  , session_serial#
  , time_waited
  , sample_time
order by 
    session_id 
  , sample_id
/

you can aggregate the result. The output is here.

I get an extract of this output to try to explain what happens.

What my anonymous PL/SQL (see the previous post) does, is to polling the V$SESSION_ACTIVE_SESSION every second and save the output in MY_ACTIVE_SESSION_HISTORY table.

Because each time, the script read the same rows plus one (the V$ASH is renewed each second), for the same sessions, you see a lot of rows for each loop

Consider, for example, the session 1732/serial 31065.

select
    sample_id
  , session_id
  , session_serial#
  , time_waited
  , count(*)
  , sample_time
from
    my_active_session_history
    where session_id=1732 and session_serial#=31065
group by
    sample_id
  , session_id
  , session_serial#
  , time_waited
  , sample_time
order by
    session_id
  , sample_id
/

Click for enlarge

It appears 50 times for the snap_id 50243482, 49 times for the snap_id 50243483 and 48 times for snap_id 50243484 (see note).

What happens to the session 1732, where for the sample_id=50243484 you see two distinct rows?

The following statement has this output (it is just the detail of the session 1732):

select 
    row_number() over (partition by sample_id order by sample_id, inserting_time) rn
  , inserting_time
  , sample_id
  , wait_time
  , time_waited
  , session_state
  , event
from
    my_active_session_history 
where 
   session_id=1732 and session_serial#=31065
order by 
   sample_id, 1
/

Because I'm interested just in the first rows of each change of "sample_id" and/or change of the "state" inside the same sample_id, I want to see only this changes


with 
  mash as (
    select 
        inserting_time
      , time_waited
      , sample_id
      , sample_time
      , session_id
      , wait_time
      , event
      , session_state
      , row_number() over (partition by sample_id, time_waited, session_state, event order by inserting_time) rn
    from 
      my_active_session_history
    where 
      session_id=1732 and session_serial#=31065
)
select 
    sample_id
  , sample_time
  , inserting_time
  , time_waited
  , session_state
  , event
from 
  mash
where 
  rn =1
order by 
    sample_id
  , inserting_time
/

Click for enlarge

What happens here?

1. At sample_id =50243482 (14:50:40) the sid 1732 is working (it's ON CPU). This means that TIME_WAITED is 0 (zero)
2. One second after (14:50:41), sample_id=50243483, the sid 1732 is still working and TIME_WAITED is still 0 (zero)
3. One second after (14:50:42), sample_id=50243484, the sid 1732 wait on direct path read event, but the TIME_WAITED column is not updated. So while the SESSION_STATE/EVENT columns report the correct state of the session, the TIME_WAITED, not yet (inserting_time=15-DEC-17 02.50.43.192604 PM). Because of this Oracle update on the same SNAP_ID (50243484) this row with the correct value of TIME_WAITED column (inserting_time=15-DEC-17 02.50.44.372309 PM).

In the next session, I try to explain this "update" in a visual way

Note

Is it correct that each next sample_id, have one less? When my PL/SQL script inserts a row the first time, it does at 15-DEC-17 02.50.40.833420 PM (sample_id=50243482). The second time, it inserts at 15-DEC-17 02.50.42.016313 PM (sample_id=50243483): it inserts two seconds after the first time. The third time it inserts at 15-DEC-17 02.50.43.192604.
Anyway, the last record for all samples is the same: 15-DEC-17 02.51.40.488761 PM. Because all samples end at the same time (MX column in the next picture), each of them count one record less.

You can run the following select that show you the min and max timestamp

select * from (
  select
      sample_id
    , session_id
    , session_serial#
    , time_waited
    , sample_time
    , inserting_time
    , max(inserting_time) over (partition by sample_id) mx
    , count(*) over (partition by sample_id) cnt
    , row_number() over (partition by sample_id order by inserting_time) rn_min
    , row_number() over (partition by sample_id order by inserting_time desc) rn_max
  from
      my_active_session_history
  where 
      session_id=1732 and session_serial#=31065
  order by
      session_id
    , sample_id)
where rn_min=1 or rn_max=1
/

Click for enlarge

Please, pay attention. While the inserting_time is the instant when the row is inserted in the MY_ACTIVE_SESSION_HISTORY, the sampling happens each second (in the previous picture I put both SAMPLE_TIME and INSERTING_TIME)

Part01
Part03

Active Session History: updating the X$ASH, part 01

Part 02
Part 03

I'm a proud member of OraPub community. Some days ago I was speaking with Kaley Crum, another member of the OraPub community.

The context was ASH (Active Session History).

He told me that at some time, Oracle does an update of X$ASH in order to update the values of the TIME_WAITED column.

I didn't trust him because my conviction was that Oracle makes just an insert in this X$.

Well, I was wrong. Kaley right. Thanks to him to learn me something new.

On this argument, what is TIME_WAITED column, and some trouble with it. Kayle wrote a post himself.

Also, and I recommend it to you, a couple of post by Craig Shallahamer. You can find them here and here. And not forget the one wrote by Alberto Dell'Era.

And don't miss the pdf of Graham Wood, slide 35, where he shows the fallacies of the TIME_WAITED column in the V$ACTIVE_SESSION_HISTORY view. This series of posts are based on this slide nr 35.

What I want to do is try to explayn myself the problem, but here, in Part 01, I want just write the test I made.


1) Prepare the enviroment

create table my_active_session_history as select * from v$active_session_history where 1=2;
 
alter table my_active_session_history add (inserting_time timestamp);


2) Run the following anonymous PL/SQL


3) Run following statement, to report the result

col INSERTING_TIME for a30
col SAMPLE_TIME for a30
set lines 120
select 
    inserting_time
  , sample_id
  , sample_time
  , session_id
  , session_serial#
  , event
  , session_state
  , time_waited
from
    my_active_session_history
order by 
    session_id
  , sample_id
  , inserting_time;

You can see the result of this last statement, here.

Part 02
Part 03

Lock Chains 01: V$SESSION

I always receive a call where a colleague tell me that a session of an application is slow.  Really that session is not slow but locked by another one.

Starting with 10g, Oracle introduced a new column in the V$SESSION performance view, that report the instance number and the session id of the blockers. In this way, the troubleshooting is more simple.

Anyway, after you query the V$SESSION you have to scroll down the result set in order to match the blocking session with the locked session.

Because the relation between the blocker and locked sessions is of "hierarchical" type, I can use the hierarchical query (session 123 is blocking the session 234 and session 345. This last one also is blocking the session 999, for example) to try to picture this relation.

*****
***** Please pay attention. The following statements are not RAC aware yet. So they work just on a single instance *****
*****

col path_state for a30
col path_event for a160
col path_sid for a20
col b_session for 999999
set lines 230
set pages 25
with
  snap as (
   select
        sid
      , blocking_instance
      , blocking_session
      , sql_id
      , prev_sql_id
      , event
      , state
    from
      v$session)
select
    to_char(sysdate, 'dd/mm/yyyy hh24:mi:ss') now
  , CONNECT_BY_ISCYCLE cycle
  , CONNECT_BY_ROOT sid b_session
  , SID l_session
  , SUBSTR(SYS_CONNECT_BY_PATH(
        DECODE(state,
                     'WAITING', sid||'/'||nvl(sql_id, prev_sql_id||'(p)')||'/'||event,
                                sid||'/'||nvl(sql_id, prev_sql_id||'(p)')||'/ON CPU'), ' -> ')
          ,5) path_event
 from
  snap
where
  level >1
connect by
  nocycle prior sid=blocking_session
/

The result is like this

Picture 01 - Click for enlarge

Picture 02 - Click for enlarge

There are a lot of things that seem to be strange.


##### Picture 01

There are sessions that lock each other. They have the column "CYCLE" to 1. Both of them are waiting for "read by other sessions" event. This is strange because it seems to be a deadlock, but there isn't a trace file containing the "ORA-00060: deadlock detected while waiting for resource".

Other curious things are the "enq: TX - row lock contention". In green, I highlighted this. The session 2330 is waiting for the session 2330 that doing...nothing.

 In blue, instead, you can see how a session waiting for "read by other session" depend from a session is reading the same block. 

##### Picture 02

Here you can see how some sessions (in red) are locked by a session that did...nothing (in orange).

In the next post, I'll show a similar statement that you can use on V$ACTIVE_SESSION_HISTOR.

Note: How to read the output

1. The column CYCLE, tell you if you have a deadlock (like in picture 01). In this case, there will be a "1"
2. B_SESSION and L_SESSION columns are the blocking session (the head) and the locked session (the tail), respectively 
3. PATH_EVENT column, show you the lock path. It includes the Session ID, the SQL ID and the wait event if that session in waiting. Otherwise, the wait event is replaced by "ON CPU" 
4. If the SQL_ID is NULL, then I try to use the PREV_SQL_ID. In this case a "(p)" will appear (for example "2sbqfhnjz6ybw(p)" in orange in the picture 02). Anyway there are some cases where also the previous SQL_ID is NULL. In this case there will be only the "(p)" without the SQL_ID (B_SESSION = 1401, in the picture 02, for example).

Using XML: Index

Part One: Introduction to XML

• Introduction, Part 01: What is this?
• Introduction, Part 02: Anatomy of a document
• Introduction, Part 03: Path

Part Two: From XML to rows and columns

• Insert, Part 01: Passing values
• Insert, Part 02: Insert via SQL*Plus
• Select, Part 01: EXTRACT and EXTRACTVALUE
• Select Part 02: XMLQuery
• Select, Part 03a: XMLTable
• Select, Part 03b: XMLTable (continued)
• Recap

Part Three: From rows and columns to XML

• XMLElement
• XMLAttributes
• XMLForest
• XMLConcat
• XMLAgg
• XMLColAttVal
• DBMS_XMLGEN.GETXMLTYPE

Using XML 17: DBMS_XMLGEN

I started this series on XML because of a query wrote by Connor McDonald. Please read my previous post.

Now there is just a last post where I want to talk about DBMS_XMLGEN.GETXMLTYPE.

First of all the definition of DBMS_XMLGEN package[1].

From [1]: The DBMS_XMLGEN package converts the results of a SQL query to a canonical XML format. The package takes an arbitrary SQL query as input, converts it to XML format, and returns the result as a CLOB. This package is similar to the DBMS_XMLQUERY package, except that it is written in C and compiled into the kernel. This package can only be run on the database.

You can find the list of procedure and functions of DBMS_XMLGEN package in [2]

The GETCMLTYPE function definition is taken from [3]: Gets the XML document and returns it as XMLType

As you can see, the first argument is a sqlQuery. So, in order to try a test, I just run a simple query.

First I a run the main statement and check the result

At this point, I run a really simple statement with DBMS_XMLGEN.GETXMLTYPE using the previous select.

So, GETXMLTYPE transforms the resultset of a select in an XML. The root node is "ROWSET" while the children are "ROW". Inside this children, there are other children: one of each column specified in the inner select.

Now I run the part of XML from my previous post (I just modify owner a table name because I run the statement on a different database):

At this point, you have all the instruments to correctly understand the Connor McDonald's select.

Reference

[1] https://docs.oracle.com/cloud/latest/db121/ARPLS/d_xmlgen.htm#ARPLS374
[2] Summary of DBMS_XMLGEN Subprograms
[3] GETXMLTYPE Functions

Part16 Index

Using XML 16: XMLColAttVal

From [1]: Oracle SQL function XMLColAttVal generates a forest of XML column elements containing the values of the arguments passed in.

select XMLElement("User",
          XMLAttributes(USER_ID as "User ID"),
          XMLColAttVal(USERNAME, ORACLE_MAINTAINED as "Is_Ora_Owned", CREATED)
       ) user_details
from all_users fetch first 3 rows only

Click to enlarge

References

[1] https://docs.oracle.com/database/122/ADXDB/generation-of-XML-data-from-relational-data.htm#GUID-F643284F-A6AF-47A9-AF2C-58EF7E88C87F

Part15 Index Part17

Using XML 15: XMLAgg

From [1]: You use SQL/XML standard function XMLAgg to construct a forest of XML elements from a collection of XML elements — it is an aggregate function.

Numeric literals are not interpreted as column positions. For example, ORDER BY 1 does not mean order by the first column. Instead, numeric literals are interpreted as any other literals.

I want just try this select

select USERNAME, XMLElement("Schemas",
   XMLAgg(XMLElement("User", username),
      XMLElement("When", to_char(CREATED, 'dd/mm/yyyy hh24:mi:ss')),
      XMLElement("Id", USER_ID),
      XMLElement("OracleOwn ", ORACLE_MAINTAINED)
      )
   ) xml_schemas
from all_users fetch first 3 rows only

Ok. It's failed. This is because I have to choose only one argument

select USERNAME, XMLElement("Schemas",
   XMLAgg(XMLElement("User", username)
      )
   ) xml_schemas
from all_users fetch first 3 rows only

So I have modify the the statement

select XMLElement("Schemas",
   XMLAgg(XMLElement("User", username)
      )
   ) xml_schemas
from all_users fetch first 3 rows only

Click to enlarge

Update

2017/Oct/11 - Modified the statement because I wrote it badly

Reference

[1] https://docs.oracle.com/database/122/ADXDB/generation-of-XML-data-from-relational-data.htm#GUID-971232BA-B52E-44CB-86DA-E182AF7E2ACB

Part14 Index Part16

Using XML 14: XMLConcat

From [1]: You use SQL/XML standard function XMLConcat to construct an XML fragment by concatenating multiple XMLType instances.

Following an example:

select USERNAME, XMLConcat(
   XMLElement("User", username),
   XMLElement("When", to_char(CREATED, 'dd/mm/yyyy hh24:mi:ss')),

   XMLElement("Id", USER_ID),
   XMLElement("OracleOwn ", ORACLE_MAINTAINED)

) xml_concat

from all_users fetch first 3 rows only

Click to enlarge

Pay attention to the "blank" in the "OracleOnw" string: it is mantained.

References

[1] https://docs.oracle.com/database/122/ADXDB/generation-of-XML-data-from-relational-data.htm#GUID-3806853D-2AAA-4D2D-AD98-3E0941F7BE17

Part13 Index Part15

Using XML 13: XMLForest

You use SQL/XML standard function XMLForest to construct a forest of XML elements.

From [1]: Each of the value expressions (value_expr in Figure 8-3) is converted to XML format, and, optionally, identifier "alias" is used as the attribute identifier. For an object type or collection, the AS clause is required. For other types, the AS clause is optional

Click to enlarge

Well, I need to rename the element:

select username,
    XMLForest(username, to_char(CREATED, 'dd/mm/yyyy hh24:mi:ss') when, 
    USER_ID, ORACLE_MAINTENED) XML_forest
from all_users fetch first 3 rows only

Click to enlarge

I also can indent the functions. In following example I use XMLElement and XMLForest

Click to enlarge

Just let me reformat the output changing the column size

Click to enlarge

and I can indent the functions

Click to enlarge

References

[1] https://docs.oracle.com/database/122/ADXDB/generation-of-XML-data-from-relational-data.htm#GUID-D9666485-0383-4AF6-B352-B9A90AE1DCFB

Part12 Index Part14

Using XML 12: XMLAttributes

From [1]: SQL/XML standard function XMLAttributes can be used together with XMLElement, to specify attributes for the generated elements. The first argument to function XMLElement defines an identifier that names the root XML element to be created. The root-element identifier argument can be defined using a literal identifier. If an attribute value expression evaluates to NULL, then no corresponding attribute is created. The data type of an attribute value expression cannot be an object type or a collection.

This is the syntax:

I run following statement

select username, XMLElement("Schema",
   XMLAttributes(username as "User",
   to_char(CREATED, 'dd/mm/yyyy hh24:mi:ss') as "When"
   USER_ID as "Id"
   ORACLE_MANTAINED as "OracleOwn")
   ) xml_user_details
from all_users fetch first 3 rows only

As you can see, the XML in yellow rectangle, is not complete. I have to format the column, changing the value of long

References

[1] https://docs.oracle.com/database/122/ADXDB/generation-of-XML-data-from-relational-data.htm#GUID-868E591C-19A1-4E4A-BC2A-013181071B77

Part11 Index Part13

Using XML 11: XMLElement

From [1]. The SQL/XML standard function XMLElement constructs XML elements from relational data. It takes as arguments an element name, an optional collection of attributes for the element, and zero or more additional arguments that make up the element content. It returns an XMLType instance.

The first argument to function XMLElement defines an identifier that names the root XML element to be created. The root-element identifier argument can be defined and, if it is defined, the identifier must not be NULL or else an error is raised.

This is the syntax:

I want to do a very simple test. I use the ALL_USERS view, in order to the run the first select.

Let's reformat the output

Ok. How you can see, the XMLElement, transform the column in a XML. Let's see more on the ALL_USERS view:

I add a second column in my select (green and red).

Because of DATE data type, I change to output using the TO_CHAR function

Really simple. But I can put an XMLElement (gray and green) inside another one (red)

So I obtain a more complex XML.

[1] https://docs.oracle.com/database/122/ADXDB/generation-of-XML-data-from-relational-data.htm#GUID-868E591C-19A1-4E4A-BC2A-013181071B77

Part10 Index Part12

Using XML 10: Comparing functions

In this post, I want just compare the EXTRACT and EXTRACTVALUE functions with the new XMLQuery and XMLTable functions, respectively.

What I want to show is summarized in this table

Old Syntax	New Syntax
EXTRACT	XMLQuery
EXTRACTVALUE	XMLTabel

But wait a moment. Really, EXTRACTVALUE extracted the value while in the previous example the XMLTable was the same as XMLQuery, they is EXTRACT function.

The example for XMLTable operator not used all parameters. I not used the "COLUMN" option.

It's the time.

SQL> SELECT rownum, t.*, tx.* from mytab t, 

  2> XMLTable('/alertlog/msg[@pid="65370"]' 

  3> PASSING t.XML_COLUMN COLUMNS message path 'msg', text path 'txt') tx

Click to enlarge

Now you can say that XMLTable is like the EXTRACTVALUE function. So the table I wrote above is more correctly like this one:

Old Syntax	New Syntax
EXTRACT(<xmltype>, <row-pattern>)	XMLQuery(<row-pattern>          PASSING <col-name>          RETURNING CONTENT)
EXTRACTVALUE(<col-name>, <row-pattern>)	XMLTabel(<row-pattern>          PASSING <expr>          COLUMNS <column> PATH <string>)

At this point I'm (almost) ready to modify a statement wrote by Connor McDonald. Yes. My goal when I decided to write these posts was to understand a particular statement that McDonald wrote in him video [1]

The statement I used is a little different: I just modified some aliases and the way to quote the string (in green the modifications)

with xml 
  as (
     select
       table_name
     , dbms_xmlgen.getxmltype(
          q'[select 
             table_name
           , partition_name
           , high_value
     from 
       dba_tab_partitions
     where 
       table_name=']'||table_name||q'[']') as xml_output
    from dba_tables t
    where table_name in ('COMPOSITE_RNG_LIST')
    and   owner='FRTD_CORP'
    )
select
   extractValue(rws.object_value, '/ROW/TABLE_NAME') table_name,
   extractValue(rws.object_value, '/ROW/PARTITION_NAME') partition_name,
   extractValue(rws.object_value, '/ROW/HIGH_VALUE') high_value
   from xml x,  
   table(xmlsequence(extract(x.xml_output, 'ROWSET/ROW'))) rws
order by 1,2,3;

Click to enlarge

What I don't discussed yet is the DBMS_XMLGEN.GETXMLTYPE. I'm going to investigate this function in the 3th part ("From rows/columns To XML" [3]) of this collection on XML.

For now, from [2], we know that "the PL/SQL package DBMS_XMLGEN creates XML documents from SQL query results". It should be enough in order to understand the meaning of the above statement.

Before to go ahead, I need to know the format of my XML source: it comes from CTE (Common Table Expression). So I run just this statement

Click to enlarge

Comparing this output with the one I used in my examples, you can build following map

McDonald case	My example
ROWSET	alertlog
ROW	msg
TABLE_NAME	txt
PARTITION_NAME	txt
HIGH_VALUE	txt

Based on all you saw untill now, I can rewrite the above statement like this one (in a red square the two core pieces)

with xml 
   as (
      select
         table_name
       , dbms_xmlgen.getxmltype(
            q'[select 
               table_name
             , partition_name
             , high_value
      from 
          dba_tab_partitions
      where 
          table_name= ']'||table_name||q'[']') as xml_output
      from dba_tables t
      where table_name in ('COMPOSITE_RNG_LIST')
      and owner='FRTD_CORP'
      )
select 
      rws.table_name
    , rws.partition_name
    , rws.high_value
from 
      xml x
    , xmltable('/ROWSET/ROW'
               PASSING
                       x.xml_output 
               COLUMNS 
                       table_name PATH 'TABLE_NAME'
                     , partition_name PATH 'PARTITION_NAME'
                     , high_value PATH 'HIGH_VALUE') rws
order by 1,2,3;

Click to enlarge

Pay attention on the case of you are searching. If you remember, XML is case sensitive, so you must specify ROWSET, ROW and others in upper case. Infact if you run the previous statement using the search in lower case you found noting:

XML is case sensitive

For semplicity, the following picture I just put together both "select" pieces about old and new syntax.

Click to enlarge

Update

2017/Sep/10 - Added the section about Connor McDonald.

2017/Sep/11 - Added the case sensitive example

Reference

[1] https://www.youtube.com/watch?v=yKHQQXKdfOM&t=82s
[2] https://docs.oracle.com/database/122/ARPLS/DBMS_XMLGEN.htm#ARPLS374

[3] http://orasal.blogspot.com/2017/09/using-xml-argument-index.html

Part09 Index Part11