Wednesday, 22 November 2017

Chatty Applications and Simple SQL

One type of "poor performance" scenario I have come across a few times is due to what I call "chatty applications". These are applications that execute a disproportionally high number of what look like very simple SQL queries for every business transaction they do. And often this is a deliberate design choice by the application architects and developers, claiming that simpler SQL statements on single tables using indexed columns always leads to efficient execution plans, so it must be better than anything else. Unfortunately it is the high volume of execution of such "simple SQL queries" that is the cause of the poor performance these applications often experience. Both in terms of poor response times for individual users, and poor total throughput across all the users. Why then are such "chatty applications" the cause of poor performance and not the solution?

First, there are network round trips needed for each SQL statement executed, and these are typically milliseconds in terms of order of magnitude of elapsed time. The SQL statement text is sent from the client to the database server, and then the resultant data sent back from the server to the client. And when there are hundreds of SQL statements being executed per business transaction, then these all add up. Executing a "business transaction" in such a "chatty application" can end up spending more time transferring data back and forth over the network than actually doing work with that data on either the client or database server system.

Second, the high SQL execution volume results in high library cache usage and contention for access to it inside Oracle. For each SQL statement executed, Oracle will check if that SQL is already in the library cache (in the shared pool in the SGA) and if so will go ahead and use the associated execution plan. Such library cache look ups involve various temporary internal locks on the library cache, to make sure it doesn't change while your session is examining it. And the more SQL statements being executed leads to more library cache lookups and more contention for the controlling locks on it.

Third, the volume of data transferred to the client and then processed on it can be much higher than if a single, more complex SQL statement was used instead. This is also about network delays, but this time it is due to the amount of intermediate result data being transferred to the client, as it works its way through data from different tables as part of its "business transaction".

Ultimately this all a variation on the "row by row" processing mind set where all the raw data is pulled back into the client, which further processes it such as joining together data from multiple tables. And a "row set" based approach can be more efficient - executing fewer but more complex SQL statements on the database server, to send only the final resultant data back to the client.

I have seen this kind of scenario in action at a number of customers, where it was their application design that was slowing down and limiting the performance of their application. In some cases we were able to identify the highest use cases in the application and successfully modify it to use fewer, more complex SQL queries instead, so that more work is done per execution by far fewer SQL statements. In other cases it was not as clear due to the application developers using a "glue layer" of third party software to handle the application to database mappings and constructing the SQL queries to be executed in each case. Fixing this requires reconfiguration of the "glue layer" to issue more complex SQL queries involving multi-table joins. However, in at least one case the customer did not at that time have enough expertise in how to use this "glue layer" to achieve that. They had just used it in an "out of the box" type fashion, and were now suffering as a consequence of that.

Probably the worst case I saw was an application using another "glue layer" that only ever issued SQL queries for a single column of data at a time of the form "select one-column from table-name where pkid_col = value", and consequently did multiple such SQL queries to get each individual column it needed. And obviously this led to horrible performance and scalability. Luckily the application had been written to use a well defined internal API for data access, which was in turn wrapped around the "glue layer". We were able to insert an extra layer between the application's API and the "glue software", intercepting the application API data access calls before the "glue software" was called. We implemented a caching layer here, based on the primary key of a record from each table, which was always used on each query. For a new record query, with a different primary key value, the API now did a "select *" into a local record buffer of the whole data record, and then returned the one individual column requested. If the primary key requested matched that already in the current record buffer, then the code just returned the individual column value directly from that, avoiding any further SQL execution and executing any code in the "glue layer".

As you might expect performance increased significantly as a result of this change - replacing the multiple single column SQL queries with just one whole record SQL query followed by in-memory, local accesses, avoiding all those network round trips. Also the net load on the Oracle database server decreased significantly, with less contention on the library cache, and the CPU and disk utilisation decreased significantly as well. Consequently the performance of all the other SQL statements being executed on that database server improved as well due to those resources being freed up. A definite "win-win" all around, by accessing the data in the database in a much more efficient way.

Thursday, 16 November 2017

Redundant Grandparent Foreign Keys and Cardinality Estimate Errors

This post is about how a slightly de-normalized database design involving redundant foreign keys to other tables can end up producing sub-optimal execution plans for queries that use those extra joins as additional filter conditions.

By default the Oracle Optimizer assumes that different columns of data in a table are independent of each other, and that their data values are not correlated with each other in any way. When a query has filter conditions on multiple columns in a table, the Optimizer will combine their filter factors together assuming they are independent of each other i.e. unless it has explicit information to the contrary. So if each filter factor was estimated to match 1% of the rows in the table, then their combined filter factor would be to match 0.01% of the rows in the table (1% of 1%).

However, if the data in these columns is in fact correlated then this filter factor estimate will be wrong. A classic example is "month of birth" and "zodiac sign". Each has only 12 possible values, and the Optimizer will assume that there are 144 possible combinations. But in fact there are only 24 possible value combinations because each zodiac sign straddles only two months. Assuming a uniform distribution amongst these 24 possible values within the data rows in the table, then each pair of values would match 1 / 24 or 4.17% of the rows in the table. However, the Optimizer will assume that a pair of values would match 1 / 144 or only 0.69% of the rows in the table. Such misestimates can make the Optimizer choose a relatively inefficient access such as using an index instead of a full table scan, due to estimating too few rows to match the filters and a too low cost for the data access.

Often this kind of correlation between data values in columns in a table occurs naturally, and there is little you can directly do about it. The "month of birth" and "zodiac sign" is one such example. However, this kind of correlation between columns can also occur as a result of how the tables have been designed, and can result in sub-optimal execution plans being produced.

One scenario is adding a redundant column to a table as a foreign key to an indirectly related table, such as a "grandparent" table that is the parent of the table's direct parent. A specific example would be something like putting the "customer / account identifier" into the "order line item" table, as a redundant copy of that in the "order" table, along with the "order identifier". Or putting both the "product main category id" and "product sub-category id" into the "product" table, when there is a hierarchy of categories.

Again, in these cases, the Optimizer will assume that such columns are independent of each other, unless it has explicit information otherwise. The danger now is that the Optimizer will produce different execution plans for what are essentially the same queries depending on whether the redundant joins are included or not.

Lets look at a solid example. I created three tables each with a parent / child relationship between them, and more rows in the child tables than the parent tables. I won't post the SQL DDL for these as it will make the post too long, but their structure and contents should be straightforward enough, and all values are uniformly distributed with an equal number of child records per parent record. There are indexes on the primary key columns, and the individual foreign key columns, but nothing else. Primary key and foreign key constraints are explicitly specified.

The tables are Grandparent (1,000 rows), Parent (100,000 rows), and Child (10,000,000 rows) i.e. 100:1 ratios, and each has a primary key column named "pkid". The Parent table has a foreign key column (gp_id) to Grandparent, while the Child table has a foreign key to Parent (of p_id) and also a redundant foreign key to Grandparent (of gp_id).

I will now execute three different but equivalent queries against these tables using different combinations of the possible foreign key joins:
  1. Only direct joins from child to parent, and then parent to grandparent
  2. As previous query plus additional (redundant) join from child to grandparent
  3. Direct from child to grandparent only, and parent table omitted from query
Each query has a filter condition on another column in the grandparent table restricting the matching rows to just 5 rows in that table.

I'm using Oracle 11gR2 on Oracle Linux:
SQL> select * from v$version;

Oracle Database 11g Enterprise Edition Release - 64bit Production
PL/SQL Release - Production
CORE Production
TNS for Linux: Version - Production
NLSRTL Version - Production

First query - direct joins between parent / child tables:
SQL> @test1


SQL_ID  fddwv6gp5m26h, child number 0
select sum ( row_count   
from child c, parent p, grandparent gp
where c.p_id = p.pkid   
  and p.gp_id = gp.pkid   
  and gp.pct05 = 1

Plan hash value: 4174412392

| Id  | Operation            | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT     |             |       |       | 25668 (100)|          |
|   1 |  SORT AGGREGATE      |             |     1 |    25 |            |          |
|*  2 |   HASH JOIN          |             | 49591 |  1210K| 25668   (1)| 00:05:09 |
|*  3 |    HASH JOIN         |             |   500 |  8500 |   244   (1)| 00:00:03 |
|*  4 |     TABLE ACCESS FULL| GRANDPARENT |     5 |    40 |     5   (0)| 00:00:01 |
|   5 |     TABLE ACCESS FULL| PARENT      |   100K|   878K|   238   (1)| 00:00:03 |
|   6 |    TABLE ACCESS FULL | CHILD       |    10M|    76M| 25398   (1)| 00:05:05 |

Predicate Information (identified by operation id):
   2 - access("C"."P_ID"="P"."PKID")
   3 - access("P"."GP_ID"="GP"."PKID")
   4 - filter("GP"."PCT05"=1)
We can see that it is estimating 5 rows to be retrieved from the Grandparent table, 500 from the join to Parent, and about 50,000 from the join to Child - operations 4, 3, and 2. This is correct given the 100:1 ratio between the rows in each table. The cost is estimated at just over 25,000, which is really just the sum of the costs of the full table scans involved.

Second query - redundant join added between Child and Grandparent:
SQL> @test2


SQL_ID  0a703yvda9z4h, child number 0
select sum ( row_count 
from child c, parent p, grandparent gp  
where c.p_id = p.pkid   
  and p.gp_id = gp.pkid   
  and gp.pct05 = 1
  and c.gp_id = gp.pkid

Plan hash value: 4140991566

| Id  | Operation                          | Name          | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT                   |               |       |       | 12271 (100)|          |
|   1 |  SORT AGGREGATE                    |               |     1 |    29 |            |          |
|   2 |   NESTED LOOPS                     |               |    50 |  1450 | 12271   (1)| 00:02:28 |
|   3 |    NESTED LOOPS                    |               | 49500 |  1450 | 12271   (1)| 00:02:28 |
|*  4 |     HASH JOIN                      |               |   500 |  8500 |   244   (1)| 00:00:03 |
|*  5 |      TABLE ACCESS FULL             | GRANDPARENT   |     5 |    40 |     5   (0)| 00:00:01 |
|   6 |      TABLE ACCESS FULL             | PARENT        |   100K|   878K|   238   (1)| 00:00:03 |
|   7 |     BITMAP CONVERSION TO ROWIDS    |               |       |       |            |          |
|   8 |      BITMAP AND                    |               |       |       |            |          |
|   9 |       BITMAP CONVERSION FROM ROWIDS|               |       |       |            |          |
|* 10 |        INDEX RANGE SCAN            | IX_CHILD_PID  |    99 |       |     2   (0)| 00:00:01 |
|  11 |       BITMAP CONVERSION FROM ROWIDS|               |       |       |            |          |
|* 12 |        INDEX RANGE SCAN            | IX_CHILD_GPID |    99 |       |    21   (0)| 00:00:01 |
|  13 |    TABLE ACCESS BY INDEX ROWID     | CHILD         |     1 |    12 | 12271   (1)| 00:02:28 |

Predicate Information (identified by operation id):
   4 - access("P"."GP_ID"="GP"."PKID")
   5 - filter("GP"."PCT05"=1)
  10 - access("C"."P_ID"="P"."PKID")
  12 - access("C"."GP_ID"="GP"."PKID")
The only difference to the previous query is the addition of the extra filter condition of "c.gp_id = gp.pkid", using the redundant join between Child and Grandparent. The row count is the same - 50,000 - because the extra filter condition is redundant and doesn't change the number of matching rows in any way. But the execution plan is completely different, because the Optimizer has assumed that the two filters on Child are independent of each other, but this is not true.

The execution plan starts the same, filtering on Grandparent to 5 estimated rows, then joining to Parent to produce 500 estimated rows (operation 4 Hash Join). Now it does 2 Nested Loops - the inner to get ROWID's from Child of matching rows, and the outer to get the data row itself for the "one" column used in the output. And this outermost Nested Loop (operation 2) is only estimated to produce 50 rows, which is clearly incorrect.

What has happened is that because the "gp_id" column in Child has 1,000 distinct values in it, the Optimizer has reduced the final row estimate by this ratio i.e. it is estimating 50 matching rows and not 50,000 matching rows. Based on this it has costed the index based access to Child to come out at 12,271 which is lower than the 25,000+ cost of the full table scan in the first execution plan.

This cost seems to be arrived at as the cost of index access for one pair of values - 21 + 2 = 23 - plus one more for the access to the data row in the table by ROWID i.e. 24 cost per Child row. For the estimated 50 Child rows this gives a total cost of 12,000, which with the costs so far to the Hash Join of 244 are very close to the reported total cost of 12,271.

In fact this execution plan will take longer to execute because it will actually be processing 50,000 Child records and not just 50, and the execution plan of the first query would actually be the better one. But it is the assumption that these two foreign key columns are independent of each other that has led the Optimizer to produce this sub-optimal plan.

Third query - remove Parent table completely from query, and join directly from Child to Grandparent:
SQL> @test4


SQL_ID  76tptvjkbx08x, child number 0
select sum ( row_count   
  from child c, grandparent gp
 where c.gp_id = gp.pkid
   and gp.pct05 = 1

Plan hash value: 2796906588

| Id  | Operation           | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
|   0 | SELECT STATEMENT    |             |       |       | 25435 (100)|          |
|   1 |  SORT AGGREGATE     |             |     1 |    15 |            |          |
|*  2 |   HASH JOIN         |             | 50000 |   732K| 25435   (1)| 00:05:06 |
|*  3 |    TABLE ACCESS FULL| GRANDPARENT |     5 |    40 |     5   (0)| 00:00:01 |
|   4 |    TABLE ACCESS FULL| CHILD       |    10M|    66M| 25403   (1)| 00:05:05 |

Predicate Information (identified by operation id):
   2 - access("C"."GP_ID"="GP"."PKID")
   3 - filter("GP"."PCT05"=1)
The result is the same as before - 50,000 rows - and the execution plan is basically the first one with Parent removed - two full table scans and a Hash Join. With only one filter condition on the Child table the Optimizer has correctly estimated the number of matching rows - 50,000 - and produced the appropriate execution plan.


Beware of database designs that include redundant foreign key columns between child and grandparent tables, or across even more levels of a relationship hierarchy, and how you write queries involving such tables. While it might seem somehow better to include all known joins between all the tables in a query, this can cause the Optimizer to misestimate the number of matching rows from a table, and to choose non-optimal access methods as a result. Rather than helping the Optimizer, adding such redundant join conditions to queries actually makes things worse. As a general rule you are better off only having joins in a query between directly related tables, and not to indirectly related ones. Otherwise, eliminate the middle, intermediate tables if possible and join directly between the relevant tables, which should produce the same results.

I saw this happen recently at a client, and removing such a redundant join condition led to the Optimizer producing more realistic row estimates and a much better execution plan, with a corresponding faster execution given the data volumes involved.

The other approach to dealing with this kind of scenario is to provide extra information to the Optimizer so that it knows when two or more columns are correlated. This can be done simply by creating an index on just those columns in the table, or by creating extended statistics on the column group. As long as this extra information is present, then the Optimizer will produce a better row estimate and a better execution plan. When I tested this with such an index on the Child table on both "p_id" and "gp_id" together, the second query with the redundant join the execution plan went back to the original one - 3 full table scans and 2 hash joins.

Also be aware that Oracle is always trying to make the Optimizer more "intelligent" so the behaviour and execution plans produced can change between versions of Oracle for the same query on the same database tables. I briefly tested these same queries on Oracle 12c and got completely different execution plans. In 12c I actually got an Adaptive Execution Plan which combined both plan variations under a parent Statistics Collector operation. Depending on how many matching rows were initially retrieved it could either do a full table scan on Parent and Child with Hash Joins, or use the indexes on the foreign key columns to do direct row lookups in Nested Loops. Combined with other feedback mechanisms in the 12c Optimizer, it is possible for the Optimizer to now detect such non-optimal execution plans and produce better ones on the next execution of the same query.