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Collecting stats takes forever after adding new index - why?

978225 Newbie
Currently Being Moderated

Hello everybody,

 

I am on Oracle 11.2.0.2. Collecting statistics for one of my tables takes extremely long (several hours), and I don't know why.

 

I am inserting about 11.000 new records into a table with 5.3 million existing records using an INSERT INTO ... SELECT ...

This insert takes a little more than 3 minutes

Then I collect the stats using :

dbms_stats.gather_table_stats('SCOTT', 'S_RMP_EVALUATION_CSC_MESSAGE', estimate_percent=>DBMS_STATS.AUTO_SAMPLE_SIZE);

This now takes 2 hours.

 

If the number of records increases, the time it takes for the stats increases as well (8 hours after 70.000 newly inserted records)

 

I did not have this problem until I recently created the index I_S_RMP_EVAL_CSC_MSG_ACTIONS, but I don't understand why this would cause a drastic change like this. Especially since inserting the records and updating the index takes only a few minutes.

 

I am posting the create statements for the table and the indexes on the table below.

 

There are about 5.3 million records in the table. The table uses about 7.8 GB of space for "regular" table data and 37.6 GB for the LOB. The indexes are using:

IndexSpace used

I_S_EVALUATION_CSC_MSG_LMID: 

152 MB

I_S_EVALUATION_CSC_MSG_IDLM:

144 MB

PK_S_RMP_EVALUATION_CSC_MESSAG: 

118 MB
I_S_RMP_EVAL_CSC_MSG_ACTIONS: 5 MB

 

 

CREATE TABLE "QQRCSBI0"."S_RMP_EVALUATION_CSC_MESSAGE" 
( "ID" NUMBER(22,0) NOT NULL ENABLE, 
  "XML_MESSAGE_TEXT" CLOB, 
  "CREATION_TIME" TIMESTAMP (6), 
  "LAST_UPDATE_TIME" TIMESTAMP (6), 
  "NEXT_UPDATE_SYNC_TS" TIMESTAMP (6), 
  "SW_VERSION_XML" VARCHAR2(100 BYTE), 
  "DWH_LM_TS_UTC" DATE DEFAULT NULL NOT NULL ENABLE, 
  CONSTRAINT "PK_S_RMP_EVALUATION_CSC_MESSAG" PRIMARY KEY ("ID")
  USING INDEX PCTFREE 10 INITRANS 2 MAXTRANS 255 COMPUTE STATISTICS NOLOGGING 
  STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
  PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT FLASH_CACHE DEFAULT CELL_FLASH_CACHE DEFAULT)
  TABLESPACE "STAGING"  ENABLE
) SEGMENT CREATION IMMEDIATE 
PCTFREE 10 PCTUSED 40 INITRANS 1 MAXTRANS 255 NOCOMPRESS LOGGING
STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
  PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT FLASH_CACHE DEFAULT CELL_FLASH_CACHE DEFAULT)
TABLESPACE "STAGING" 
LOB ("XML_MESSAGE_TEXT") STORE AS BASICFILE (
  TABLESPACE "STAGING" ENABLE STORAGE IN ROW CHUNK 8192 RETENTION 
  NOCACHE LOGGING 
  STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
  PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT FLASH_CACHE DEFAULT CELL_FLASH_CACHE DEFAULT)
) ;
CREATE INDEX "QQRCSBI0"."I_S_EVALUATION_CSC_MSG_IDLM" ON "QQRCSBI0"."S_RMP_EVALUATION_CSC_MESSAGE" ("ID", "DWH_LM_TS_UTC") 
  PCTFREE 10 INITRANS 2 MAXTRANS 255 COMPUTE STATISTICS 
  STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
  PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT FLASH_CACHE DEFAULT CELL_FLASH_CACHE DEFAULT)
  TABLESPACE "STAGING" ;
CREATE INDEX "QQRCSBI0"."I_S_EVALUATION_CSC_MSG_LMID" ON "QQRCSBI0"."S_RMP_EVALUATION_CSC_MESSAGE" ("DWH_LM_TS_UTC", "ID") 
  PCTFREE 10 INITRANS 2 MAXTRANS 255 COMPUTE STATISTICS NOLOGGING 
  STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
  PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT FLASH_CACHE DEFAULT CELL_FLASH_CACHE DEFAULT)
  TABLESPACE "STAGING" ;
CREATE BITMAP INDEX "QQRCSBI0"."I_S_RMP_EVAL_CSC_MSG_ACTIONS" ON "QQRCSBI0"."S_RMP_EVALUATION_CSC_MESSAGE" (DECODE(INSTR("XML_MESSAGE_TEXT",'<actions>'),0,0,1)) 
  PCTFREE 10 INITRANS 2 MAXTRANS 255 COMPUTE STATISTICS 
  STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
  PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT FLASH_CACHE DEFAULT CELL_FLASH_CACHE DEFAULT)
  TABLESPACE "STAGING" ;

What causes this extreme long duration for collecting the stats, and what can I do to fix that (apart from removing the index again)? Why does collecting the stats take so much longer than updating data and indexes?

 

Thank you...

  • 1. Re: Collecting stats takes forever after adding new index - why?
    sb92075 Guru
    Currently Being Moderated

    if you enable SQL_TRACE, then you'll see where actual time is being spend & can even generate EXPLAIN PLAN for all impactful SQL statements

  • 2. Re: Collecting stats takes forever after adding new index - why?
    Hemant K Chitale Oracle ACE
    Currently Being Moderated

    >CREATE BITMAP INDEX "QQRCSBI0"."I_S_RMP_EVAL_CSC_MSG_ACTIONS" ON "QQRCSBI0"."S_RMP_EVALUATION_CSC_MESSAGE" (DECODE(INSTR("XML_MESSAGE_TEXT",'<actions>'),0,0,1))

     

    That's strange.  You are creating (a) a Bitmap Index  on (b) a Function  on (c) an out-of-line LOB.

    Why do you want to index the LOB ?  Can you store the desired "flag"  (whether "<actions>" exists or not) in a separate column and then index it ?

    Hemant K Chitale

  • 3. Re: Collecting stats takes forever after adding new index - why?
    Dave Rabone Journeyer
    Currently Being Moderated

    "I am inserting about 11.000 new records into a table with 5.3 million existing records"

     

    ... which is adding 0.2% of the existing rows, so I would not expect the statistics to change significantly, certainly not to the extent that the optimiser would make different decisions. It might be different if you were doing something funky with histograms, but you aren't.

     

    As many have said previously "The quickest way to do something is not to do it at all"

  • 4. Re: Collecting stats takes forever after adding new index - why?
    978225 Newbie
    Currently Being Moderated

    Dave Rabone wrote:

     

    "I am inserting about 11.000 new records into a table with 5.3 million existing records"

     

    ... which is adding 0.2% of the existing rows, so I would not expect the statistics to change significantly, certainly not to the extent that the optimiser would make different decisions. It might be different if you were doing something funky with histograms, but you aren't.

     

    As many have said previously "The quickest way to do something is not to do it at all"

    Are you saying recollecting the stats is not necessary in the first place?

     

    That may be true for just doing the insert once, but after doing this often enough, I will be so far off that I do need to recollect stats, and then I'll be back to my original problem, won't I?

  • 5. Re: Collecting stats takes forever after adding new index - why?
    rp0428 Guru
    Currently Being Moderated

    A couple of other anomalies:

     

    Why do you have DEFAULT NULL on a column with a NOT NULL constraint?

    "DWH_LM_TS_UTC" DATE DEFAULT NULL NOT NULL ENABLE,   

    Why are you creating two composite indexes on the same two columns?

    "S_RMP_EVALUATION_CSC_MESSAGE" ("ID", "DWH_LM_TS_UTC")   

    "S_RMP_EVALUATION_CSC_MESSAGE" ("DWH_LM_TS_UTC", "ID")  

    The ID column is already a primary key. So if you provide ID only it will use that index. And if you provide both column values then either of the two indexes will do the job.

  • 6. Re: Collecting stats takes forever after adding new index - why?
    978225 Newbie
    Currently Being Moderated

    Hemant K Chitale wrote:

     

    Why do you want to index the LOB ?  Can you store the desired "flag"  (whether "<actions>" exists or not) in a separate column and then index it ?

    Hemant K Chitale

    I figured that's what functional indexes are for. And bitmap because it's only 2 possible values, so a regular index wouldn't perform.

    I will try your suggestion as a workaround for my problem, but I would still need the bitmap index and I would still have to search through the LOB's value, so it's mostly still the same thing.

     

    Any idea why this is happening in the first place?

  • 7. Re: Collecting stats takes forever after adding new index - why?
    Hemant K Chitale Oracle ACE
    Currently Being Moderated

    >And bitmap because it's only 2 possible values

    If the distribution among the two values (0 and 1) would be equal (50% each), a bitmap index wouldn't really help.  A bitmap index on such a column can be useful if it can be combined with other bitmap indexes on the same table --- which doesn't apply to this table.

    If, however, the distribution is, say 90-10 and you are querying for the 10% of rows, a bitmap index can be helpful.

     

    You suspect (or have identified) that the gather_stats has been taking much longer since the introduction of this new index.  Can you evaluate your existing SQL executions to see if the index is being used at all ?  SQL statements must use the same set of decode(instr...) functions to be able to use the index.

    A function based index actuall adds a new virtual column to the table gather_table_stats has to read that column.  I wonder how Oracle performs a gather_stats on a function on a LOB.  Never seen it before.

     

     

    Hemant K Chitale


  • 8. Re: Collecting stats takes forever after adding new index - why?
    978225 Newbie
    Currently Being Moderated

    Why do you have DEFAULT NULL on a column with a NOT NULL constraint?

    "DWH_LM_TS_UTC" DATE DEFAULT NULL NOT NULL ENABLE,  

    Because I need it NOT NULL and because the only DEFAULT value that would make sense for this case is the result of a user-defined function, which Oracle will not accept as DEFAULT. So this way, I basically force any INSERT to specify a value. If not, the INSERT will fail.

     

    Why are you creating two composite indexes on the same two columns?

    "S_RMP_EVALUATION_CSC_MESSAGE" ("ID", "DWH_LM_TS_UTC")  

    "S_RMP_EVALUATION_CSC_MESSAGE" ("DWH_LM_TS_UTC", "ID"

    I was trying to see if any of these two indexes would be picked up by the queries I am trying to optimize. I will most likely end up removing at least one if not both.

  • 9. Re: Collecting stats takes forever after adding new index - why?
    978225 Newbie
    Currently Being Moderated

    Hemant K Chitale wrote:

     

    >And bitmap because it's only 2 possible values

    If the distribution among the two values (0 and 1) would be equal (50% each), a bitmap index wouldn't really help.  A bitmap index on such a column can be useful if it can be combined with other bitmap indexes on the same table --- which doesn't apply to this table.

    If, however, the distribution is, say 90-10 and you are querying for the 10% of rows, a bitmap index can be helpful.

    It's more like 90/10 and I am querying for the 90 part, but did it this was because I was thinking the index was going to once evaluate the formula and then store the result in the index structure - basically exactly what your original suggestion with the extra flag column would do, but stored differently.

     

    You suspect (or have identified) that the gather_stats has been taking much longer since the introduction of this new index.  Can you evaluate your existing SQL executions to see if the index is being used at all ?  SQL statements must use the same set of decode(instr...) functions to be able to use the index.

     

    Yes, the query is using exactly the same expression as the index.

     

    A function based index actuall adds a new virtual column to the table gather_table_stats has to read that column.  I wonder how Oracle performs a gather_stats on a function on a LOB.  Never seen it before.

    Maybe this is where my disconnect is - when is the index's function evaluated? Is it when the index is updated after insert/update of a record, or is it during query time? I would have expected the first, meaning that evaluation of the function should already have happened when I ask Oracle to collect the stats. I would not expect Oracle to evaluate the function for each newly inserted row as part of collecting the stats?!?! Or am I wrong?



  • 10. Re: Collecting stats takes forever after adding new index - why?
    Hemant K Chitale Oracle ACE
    Currently Being Moderated

    The function should be evaluated when the index is created and when rows are inserted/updated.

    However, I am stumped by your using it against a LOB.

     

    What sort of queries do you run against the table ?  If the queries are retrieving 90% of the rows and you are retrieving columns other than the return value of the function, Oracle has to go to the table and not use the index.

     

     

    Hemant K Chitale

  • 11. Re: Collecting stats takes forever after adding new index - why?
    Jonathan Lewis Oracle ACE Director
    Currently Being Moderated

    Hemant has given you the clue.

     

    When you create a function-based index Oracle supports it with a virtual column.

    When you gather table stats Oracle evaluates the function for each row in the table - it doesn't try to look at the index to derive the statistics from the index (which, technically, would be possible).

    Your extra time for collecting stats is probably spent hacking through all those LOBs.

    This might be a case where you should add a column and store the result of the function (and then not bother with an index on the column).

     

    Regards

    Jonathan Lewis

  • 12. Re: Collecting stats takes forever after adding new index - why?
    978225 Newbie
    Currently Being Moderated

    However, I am stumped by your using it against a LOB.

    Well, it's just a column. It may contain larger content and be stored differently, but still - it's just a column... Isn't it?

     

     

    What sort of queries do you run against the table ?  If the queries are retrieving 90% of the rows and you are retrieving columns other than the return value of the function, Oracle has to go to the table and not use the index.

    The content of the LOB is XML which is later inserted into an object-relational XmlType column. There are some XMLs that contain a tag (<actions>) which violates the structure, thus I am trying to filter those out. I was using an XPath expression for that but figured that a plain text search should be faster, because that will save the engine the effort of parsing the whole thing in some kind of an XML structure.

    The query is basically a

    INSERT INTO ... SELECT ... from S_RMP_EVALUATION_CSC_MESSAGE
    WHERE DECODE(INSTR("XML_MESSAGE_TEXT",'<actions>'),0,0,1)=0

     

    Then I figured I add the function-based BITMAP index to prevent Oracle from having to re-evaluate at query time and instead just use the pre-calculated index value.

    You are right, it is not a covering index, so Oracle still needs to access the table data to get the other columns for the INSERT, but I figured it would use the index to select the matching records first and then move on to the records in the table. At that point, a lot would have been gained already, compared to evaluating the WHERE condition during query time.

  • 13. Re: Collecting stats takes forever after adding new index - why?
    978225 Newbie
    Currently Being Moderated

    When you create a function-based index Oracle supports it with a virtual column.

    Ok, so far so good. So the virtual column holds the result of the function.

     

    When you gather table stats Oracle evaluates the function for each row in the table - it doesn't try to look at the index to derive the statistics from the index (which, technically, would be possible).

    This is what I did not expect. If it already has it evaluated and stored in the virtual column, I would have expected it to simply read it from there rather than re-evaluating the function for each record... Are you sure about this?

    Do you happen to know if the same thing is going to happen with a user-declared virtual column? I would be creating the same virtual column as Oracle is doing internally... Guess I'll try that...

    Your extra time for collecting stats is probably spent hacking through all those LOBs.

    This might be a case where you should add a column and store the result of the function (and then not bother with an index on the column).

    This would be an explanation for why things take so long. Although I don't see why Oracle wouldn't simply be using the value from the virtual column.

  • 14. Re: Collecting stats takes forever after adding new index - why?
    Jonathan Lewis Oracle ACE Director
    Currently Being Moderated

    It's a VIRTUAL column - Oracle doesn't store the value of the function in the table for each row, it simply stores the description of the defining function as the column definition in the data dictionary.  The index DOES hold the actual calculated value (with the rowid of the row that each value came from), but Oracle doesn't have a mechanism for looking at the index when it's gathering column stats.  (In fact such a mechanism could only be created for a single column index anyway).

     

    Regards

    Jonathan Lewis

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