Thursday, 11 January 2018

Hash Join Overflow Cost Formula #1

Introduction


The Hash Join join method was introduced in Oracle version 7 (7.3 specifically I believe), and one of its main goals was to be a method that lent itself well to being parallelisable. However, it is such an efficient join method for larger data volumes even in serial execution that it is often picked by the Optimizer over Nested Loops or Sort Merge because of its lower execution cost. This makes the Hash Join method probably the most frequently used method by the Oracle Optimizer, often appearing in execution plans for SQL queries.

A Hash Join works by building a table in memory containing the data from the first data set (termed the Build data set), and then reading the second data set (the Probe data set) to lookup each data row into the in-memory table for any match (join). The in-memory table is structured and accessed by applying a "hash function" to the relevant join data columns. This hashing has various benefits around performance, handling any kind of input data value range, and distributing the input values within the table (minimising bunching values together).

When this hash table fits in memory the additional cost of the Hash Join operation is negligible because it only involves CPU and memory operations and these are multiple orders of magnitude faster than disk accesses i.e. there is often little or no difference to the total reported cost of the Hash Join over the sum of the costs of access of each source data set. However, when the hash table needed is larger than can fit in available memory in the PGA then it must overflow to disk, which in turn significantly increases the cost of the Hash Join operation itself.

A question I have had for a long time is "How does Oracle cost this overflowing Hash Join operation"? Can I replicate this cost formula and understand what the main factors are within this reported cost? Which is a bigger factor - the size of the Build data set or the Probe data set? Knowing such things it might offer the possibility of gaining some insights into ways of tuning such large hash joins. At the least I would know more about how the overflowing hash join actually works in practice.

Jonathan Lewis gives a description in his Cost Based Oracle Fundamentals book of how the overflowing Hash Join operation works, with a formula for the major cost components involved. However, I have always found this formula to be more descriptive than quantitative, and to not be easy to use to arrive at a comparable value to what the Optimizer has reported.

I would like a more straightforward quantitative formula that I could use myself to estimate whether a Hash Join will overflow to disk or not, and how much its cost will be. After some research I believe I have arrived at such a formula which I will share here. Note that I'm not saying this is a "perfect" formula, just that this is the conclusion I have arrived at so far as a result of the tests I have done, and it seems to fit the results I have very well. I'll continue to post more details when I refine or revise this in the future

Why Hash Join Overflows


The limit to the size of a single Hash Table or other "work area" in the PGA is determined by a hidden, internal initialization parameter of "_smm_min_size". If the size of the Hash Table needed would be larger than this, then the Optimizer assumes that it will overflow to disk and costs it accordingly. My notes say that the value of "_smm_min_size" is the larger of 128 KB or 0.1% of PGA_AGGREGATE_TARGET. I cannot find exactly where I got this information from, but my memory is telling me that it was from Randolf Geist, possibly in a response to a question on one of the Oracle forums.

The main reason Oracle limits the potential size of a Hash Table is to ensure that a high number of other queries can run concurrently and not be starved of memory within the PGA. The Optimizer is assuming a worst case scenario to make sure other sessions do not suffer when a very large query is executed by one session. However, it is possible that when executed the hash table will not overflow to disk i.e. if at the moment that query is executed there is enough free memory in the PGA, then Oracle will let that session have a larger "work area" than the value of "_smm_min_size". So even though the Optimizer has costed the Hash Join operation as an overflow to disk and costed it accordingly, it does not mean that it will always overflow to disk when executed.

How Overflow Hash Join Works


Jonathan Lewis gives a description in his book of how the Hash Join works when it overflows to disk. I won't repeat the details here as they are not particularly relevant at the end of the day. But a crude summary would be that:
  • The first data set is read once and broken into "chunks" that are written out to disk (temporary storage), where each chunk is a contiguous sub-set of the overall hash table
  • One or more of these chunks are then kept in memory ready for the pass over the second data set
  • The second data set is read in and hashed in the normal way:-
    • If it hashes to an in-memory chunk then it is matched as normal
    • Otherwise it is written out to disk (temporary storage) split out into chunks on the same basis as the first data set
  • Then remaining chunks of the first data set are read into memory, and the corresponding chunks of the second data set read again and matched as normal. This is repeated until all of both data sets have been processed.
Essentially this is saying that there will be an additional pass over both sets of data - after the first read of each data set (already costed within the execution plan), there is an additional write out to disk of the majority of each data set, followed by a read back of each data set. Also extra reads and writes may be needed in the first pass of each data set, to keep the data in the "chunks" properly grouped together on disk.

It is not clear whether these write and read operations will be single block or multi-block disk read operations, or how many of them there will be. Potentially multi-block reads could be used when reading back in the pre-hashed chunks from disk. Luckily though this turns out to be irrelevant to the cost formula I have arrived at.

Deriving Hash Join Overflow Cost


Here is how I went about it. I created a set of standard test tables (see later for SQL DDL), each with a mix of NUMBER and VARCHAR2 columns to pad them out a bit, and populated them using a repeatable "connect by" data generator with a different number of rows in each test table. I then ran a query joining 2 of these tables together (see later for SQL), immediately examined the execution plan (from dbms_xplan.display_cursor) and noted the costs of each operation.

Without any indexes on any of these tables the execution plan was always 2 Full Table Scans feeding into a Hash Join operation. When the smaller, build table became large enough the Hash Join operation would overflow to disk, causing its cost to rise significantly, and a "TempSpc" column to appear in the execution plan with a reported value.

By varying only one thing at a time between queries I could see how the Hash Join cost changed when it was overflowing to disk. I was not interested in those executions where the Hash Join did not overflow to disk i.e. where the hash table did fit in memory. Only those executions that involved the Optimizer assuming it would overflow to disk. By examining the change in cost for the Hash Join operation for a corresponding change in only one of the joined tables I could deduce a multiplying factor being used within the underlying Hash Join cost calculation.

My Oracle version is 12.1 on Oracle Linux, so my results are only guaranteed to be accurate for that version. I would assume the results should be the same for 11g, as I don't think anything significant has changed in how the Hash Join operation is costed, but that would need to be verified.
BANNER
--------------------------------------------------------------------------------
Oracle Database 12c Enterprise Edition Release 12.1.0.2.0 - 64bit Production
PL/SQL Release 12.1.0.2.0 - Production
CORE 12.1.0.2.0 Production
TNS for Linux: Version 12.1.0.2.0 - Production
NLSRTL Version 12.1.0.2.0 - Production

Hash Join Overflow Formula and its Accuracy


I started by comparing the Hash Join cost when the number of columns in a data set changed i.e. the size of the data set increased for the same number of rows. Jonathan Lewis states that the memory needed per column is the storage size of the column itself plus 2 additional bytes. I observed that the Hash Join cost changed by a factor of 0.0475 per byte per 1000 rows. And that this multiplying factor was the same under different row counts in either table in the query.

This only involves the columns needed by the query itself, which the Optimizer must extract and process, and not all of the columns in the table. In this case it is the columns referenced in the "SELECT" list and those referenced in the "WHERE" clause. And the "storage size" is the number of bytes that Oracle uses to store that data on disk, which is not necessarily 1 byte per value or 1 byte per character or digit. For instance, a NUMBER is stored as 2 digits per byte.

My other observation was that when only the row count in one table changed the Hash Join cost changed by a factor of 0.5675 per 1000 rows. As this was a constant per row I wondered if this was something to do with some extra data per row causing extra disk I/Os. And 0.5675 divided by 0.0475 gives 11.9474 which is almost 12, implying a 12 byte overhead per data row within the Hash Table.

Based on this, I arrived at the following formula for an overflowing Hash Join cost:
  • ( ((Build Columns Size + 12) * Build Row Count) + ((Probe Columns Size + 12) * Probe Row Count) ) * 0.0475
Where the "Columns Size" is the sum of the hash table storage for each column i.e. data storage + 2 bytes per column.

I then checked the calculated costs from this formula against the series of test queries I had been using, and the resultant cost for the overflowing Hash Join came out almost the same in all cases. The percentage difference was under 1% in almost all cases, which I take to be a high degree of accuracy. The only anomalies are for when the "build data set" is only just bigger than can fit in the PGA, but even then it is only a 3% difference. As the size of the build data set increased so the percentage difference decreased.

On the one hand it might not be surprising to some people that my derived formula produces the same results using the same inputs that were used to create the formula in the first place. However, given that the queries I tested varied both the number of columns being selected, the number of columns being joined on, and the number of rows in each table, these would appear to cover the only variables relevant to this formula. And in each of these cases the change in the reported Hash Join cost from Oracle was always a multiplier of this fixed constant of 0.0475.

Other Factors


While I do believe that this formula is true and valid for the system I was testing on, it may not be true for all other systems. It is likely that the multiplying factor of 0.0475 will be different on other systems. Given that this additional cost for the overflowing Hash Join is due to the additional disk I/Os involved, then it would seem likely that changes to the system statistics inside Oracle for disk read times would result in a change in the value of this multiplying factor. I will investigate this in my next series of tests.

There may or may not be some small "constant cost value" involved as well within the formula, for some kind of constant overhead within the overflowing Hash Join operation. This "constant cost value" would become negligible at higher data volumes compared to the costs for the build and probe data sets, but it might explain the slightly larger difference in calculated cost at the smallest overflowing data set size.

There is also the concept of "one pass" and "multi-pass" hash joins within Oracle, as well as "optimal" hash joins. I don't understand the real difference between these, other than "optimal" is when it fits in memory and the other two are when it overflows to disk. It is possible that what I've seen has been the cost for "one pass" overflowing hash joins, and for even larger data sets a "multi-pass" hash join would be used that would involve a different cost formula.

The SQL for the table and query


Here is the SQL to create one example table - they are all the same but for name and row counts - and the query used.

Create Table - run from SQL*Plus with 2 command line arguments of the row count and a table name suffix e.g. "@crhjtab 1000000 100k".
create table hj&2
tablespace testdata
as
select r pid
     , 1 one
     , 2 two
     , 3 three
     , 4 four
     , 5 five
     , 10 ten
     , trunc (r / 10) per10
     , trunc (r / 100) per100
     , mod (r, 10) mod10
     , mod (r, 100) mod100
     , mod (r, 1000) mod1000
     , mod (r, 10000) mod10000
     , mod (r, 100000) mod100000
     , 'ABCDEFGHIJKLMNOPQRSTUVWXYZ' filler1
     , 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz' filler2
     , 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz' filler3
  from (select rownum r
          from (select rownum r from dual connect by level <= 1000) a,
               (select rownum r from dual connect by level <= 1000) b,
               (select rownum r from dual connect by level <= 1000) c
         where rownum <= &1) 
;
exec dbms_stats.gather_table_stats (user, upper ('hj&2') )
Query - run from SQL*Plus with 2 command line arguments of table name suffixes e.g. "@hj0 100k 200k"
select /* HashTest0 */ sum (hj1.one) sum_b1
  from hj&1 hj1, hj&2 hj2
 where hj1.pid = hj2.per10 
   and hj1.mod10 = hj2.mod100 ;
--
select * from table (dbms_xplan.display_cursor) ;
I've only shown one query here, as the others I used are almost the same but for the columns in the "SELECT" list. The variations of this query had different numbers of columns in the "SELECT" list, to increase the number of columns from the build and/or the probe tables.

Wednesday, 3 January 2018

Reading AWR Reports #2 - Report Overview

In the first post on Reading AWR Reports I made the point that you should first be clear on the details of the "performance problem" you are investigating. If there is no specific problem reported then there is no point looking for something that is not there in an AWR report. I also stated that an AWR Report is one amongst several tools available to you that you can use to investigate Oracle database performance problems, and you should make sure that a database wide AWR Report is the most suitable tool to be using for the specific performance problem you are currently tasked with investigating.

Assuming that has all been addressed, then the first thing I do with an AWR Report is a few high level checks - a kind of sanity check to get a feel for whether there might be some kind of performance issue there or not.

A performance problem is normally when a task takes too long to complete, and that is affected by the various resources it uses while doing its work. The key measurements are therefore both sides of this - the input work requests, and the system resource usage while doing that work. I basically want to check:
  • The resource usage as a percentage of system capacity i.e. utilisation
  • The amount of work requested / done (assuming they are the same), which is really SQL statements executed for a database
  • Amount of "wait time" within "SQL execution time" i.e. wait as percentage of work time
  • Top wait events to see how these correlate with the previous observations
That's it. Nothing more detailed for the first pass. If the input workload is high enough and the resource utilisation high enough and something looks wrong then I follow up with a second pass of the AWR Report diving down into more details based on what the first pass showed up.

Lets use the following AWR Report to show what I mean:
WORKLOAD REPOSITORY report for

DB Name         DB Id    Instance     Inst Num Startup Time    Release     RAC
------------ ----------- ------------ -------- --------------- ----------- ---
O12DB         3429470280 o12db               1 05-Dec-17 06:59 12.1.0.2.0  NO

Host Name        Platform                         CPUs Cores Sockets Memory(GB)
---------------- -------------------------------- ---- ----- ------- ----------
xxxxxxxxxx.local Linux x86 64-bit                    2     2       1       7.80

              Snap Id      Snap Time      Sessions Curs/Sess
            --------- ------------------- -------- ---------
Begin Snap:        79 05-Dec-17 09:53:43       142       3.0
  End Snap:        80 05-Dec-17 10:03:44       142       4.1
   Elapsed:               10.03 (mins)
   DB Time:                0.39 (mins)

Top 10 Foreground Events by Total Wait Time
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                           Total Wait       Wait   % DB Wait
Event                                Waits Time (sec)    Avg(ms)   time Class
------------------------------ ----------- ---------- ---------- ------ --------
DB CPU                                           12.4              53.8
log file sync                        2,727        9.3       3.42   40.4 Commit
db file sequential read                177        1.2       6.82    5.2 User I/O
control file sequential read           827         .1       0.12     .4 System I
The snapshot duration was 10 minutes, and the system had 2 CPU Cores, so there were 20 minutes of CPU capacity available. The database processing time is reported as "DB Time" and is 0.39 minutes, which is about 1.95% resource utilisation. From this I can see that the database was doing very little work at all, so there is not a database wide performance problem worth investigating. Even though the Top Wait Events seem to show that 40% of the time was spent waiting on the "log file sync" event, the actual amount of Time is very trivial - 9.3 seconds of waiting out of a 600 second period (10 minutes). Such low workloads and resource utilisation can lead to various measurement anomalies, so it is not worth trying to drill down further into the wait event details. Any actual problem is probably specific to a single session, and should be investigated using session specific tools.

Another AWR Report:
WORKLOAD REPOSITORY report for

DB Name         DB Id    Instance     Inst Num Startup Time    Release     RAC
------------ ----------- ------------ -------- --------------- ----------- ---
O12DB         3429470280 o12db               1 05-Dec-17 06:59 12.1.0.2.0  NO

Host Name        Platform                         CPUs Cores Sockets Memory(GB)
---------------- -------------------------------- ---- ----- ------- ----------
xxxxxxxxx.local Linux x86 64-bit                    2     2       1       7.80

              Snap Id      Snap Time      Sessions Curs/Sess
            --------- ------------------- -------- ---------
Begin Snap:        71 05-Dec-17 07:55:35        39       1.8
  End Snap:        72 05-Dec-17 08:05:39        40       2.0
   Elapsed:               10.06 (mins)
   DB Time:                9.35 (mins)

Load Profile                    Per Second   Per Transaction  Per Exec  Per Call
~~~~~~~~~~~~~~~            ---------------   --------------- --------- ---------
      Redo size (bytes):       3,470,831.7           5,403.2
  Logical read (blocks):          61,507.4              95.8
          Block changes:          20,563.9              32.0
             User calls:             495.9               0.8
           Parses (SQL):             302.6               0.5
         Executes (SQL):          13,287.8              20.7
           Transactions:             642.4

Top 10 Foreground Events by Total Wait Time
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                           Total Wait       Wait   % DB Wait
Event                                Waits Time (sec)    Avg(ms)   time Class
------------------------------ ----------- ---------- ---------- ------ --------
DB CPU                                          372.9              66.5
log file sync                      272,633      196.8       0.72   35.1 Commit
db file sequential read                472        2.2       4.75     .4 User I/O
Again, same database instance and a 10 minute period of time but a different workload. Now we see that the resource utilisation is up at 46.47%, being 9.35 minutes of DB Time out of 20 minutes of potential CPU capacity. From the Load Profile we can see that the database was executing over 13,000 SQL statements per second. From the wait events we see that "DB CPU" is 66.5% of the "DB Time" i.e. doing real work. So waiting as a percentage of SQL execution time would be about 33.5%. And we can see that almost all of this wait time is due to a single wait event - "log file sync".

Again, this does not directly tell us whether there is a performance problem or not. In all databases doing work there will be some wait events occurring, and one of these will always be top when ordered by wait time. That does not mean that it is necessarily a problem, or that it can be "fixed" in any way. In this case the workload is doing a lot of inserts and updates in small transactions, so there is a high frequency of commits which all involve writes to the redo logs. And we would expect the "log file sync" wait event to be there in the set of top 10 wait events in this scenario i.e. it is not a "problem" but a "result" of the workload running on the database.