People ask me from time to time what are some good SQL tuning books. It’s a tough question to answer. There are some seminal books such as “SQL Tuning” by Dan Tow and “Cost Based Optimizer Fundamentals” by Jonathan Lewis, but these are advanced books that few I know have actually read from cover to cover. If you are looking for practical SQL tuning cookbooks you might want something a little less advanced. For a practical approach, I love “Refactoring SQL Applications” by Stephane Faroult which is an easy read, again, it’s not a straight forward SQL tuning book.
Finally there is a book that provides a pragmatic down to earth approach to tuning SQL queries : “Oracle Performance Tuning and Optimization : It’s all about the Cardinalities” by Kevin Meade.
I’ve been meaning to put together a general review but as that’s a lot to tackle I thought I’d go through some of the material in the book chapter by chapter.
Chapter 1 lays out “Driving Table and Join Order”
Meade lays out the 4 parts of a query as
- driving table
- join order
- access method
- join method
This is a great way to look at query optimization. As I’ve written about before with Visual SQL Tuning, based on Dan Tow’s work, the join order is the most important part of query optimization and the first step in join order is finding the driving table. The goal is
“Remove as many rows as possible as early as possible in query execution”
Big question is how do we remove as many row as possible as early as possible in the execution? We do that by finding the best driving table. The best driving table is the table who has the predicate filter that eliminates the most rows. To find the best predicate filter we have to go through the query and find which tables have predicate filters and then determine how many rows are returned after the predicate filter verses the rows in the table. Calculating these filter ratios can be a good bit of busy work. I like my busy work automated, so when I was at Embarcadero, I worked with our DB Optimizer tool team to do this automatically. It was pretty cool. Below is a diagram produced from a query that Jonathan Lewis put together for an article on how to analyze a query visually.
The blue numbers to the bottom right of certain tables are the filter ratios. A low ratio means that a low percentage of the table is returned after applying the predicate filter.
How do you calculate these predicate filters in an automated way without DB Optimizer. One of my favorite things about Chapter 1 is a query that Kevin Meade wrote to automatically calculate predicate filter ratios.
For example if I run
SQL> explain plan for SELECT /*+ gather_plan_statistics */ order_line_data FROM customers cus INNER JOIN orders ord ON ord.id_customer = cus.id INNER JOIN order_lines orl ON orl.id_order = ord.id INNER JOIN products prd1 ON prd1.id = orl.id_product INNER JOIN suppliers sup1 ON sup1.id = prd1.id_supplier WHERE cus.location = 'LONDON' AND ord.date_placed BETWEEN sysdate - 7 AND sysdate AND sup1.location = 'LEEDS' AND EXISTS ( SELECT NULL FROM alternatives alt INNER JOIN products prd2 ON prd2.id = alt.id_product_sub INNER JOIN suppliers sup2 ON sup2.id = prd2.id_supplier WHERE alt.id_product = prd1.id AND sup2.location != 'LEEDS' ) ;
Then I run
I’ll see some output like
with frp_data as ( select ' 11' id,'DELPHIXDB' table_owner,'PRODUCTS' table_name,'PRD2' table_alias,1999 num_rows,count(*) rowcount,1999 cardinality,cast(null as number) filtered_cardinality from DELPHIXDB.PRODUCTS PRD2 union all select ' 17' id,'DELPHIXDB' table_owner,'CUSTOMERS' table_name,'CUS' table_alias,14576 num_rows,count(*) rowcount,49 cardinality,count(case when "CUS"."LOCATION"='LONDON' then 1 end) filtered_cardinality from DELPHIXDB.CUSTOMERS CUS union all select ' 2' id,'DELPHIXDB' table_owner,'SUPPLIERS' table_name,'SUP1' table_alias,99 num_rows,count(*) rowcount,50 cardinality,count(case when "SUP1"."LOCATION"='LEEDS' then 1 end) filtered_cardinality from DELPHIXDB.SUPPLIERS SUP1 union all select ' 8' id,'DELPHIXDB' table_owner,'SUPPLIERS' table_name,'SUP2' table_alias,99 num_rows,count(*) rowcount,50 cardinality,count(case when "SUP2"."LOCATION"<>'LEEDS' then 1 end) filtered_cardinality from DELPHIXDB.SUPPLIERS SUP2 union all select ' 14' id,'DELPHIXDB' table_owner,'PRODUCTS' table_name,'PRD1' table_alias,1999 num_rows,count(*) rowcount,1999 cardinality,cast(null as number) filtered_cardinality from DELPHIXDB.PRODUCTS PRD1 union all select ' 18' id,'DELPHIXDB' table_owner,'ORDERS' table_name,'ORD' table_alias,71604 num_rows,count(*) rowcount,17898 cardinality,count(case when "ORD"."DATE_PLACED">=SYSDATE@!-7 AND "ORD"."DATE_PLACED"<=SYSDATE@! then 1 end) filtered_cardinality from DELPHIXDB.ORDERS ORD union all select ' 9' id,'DELPHIXDB' table_owner,'SUPPLIERS' table_name,'SUP2' table_alias,99 num_rows,count(*) rowcount,99 cardinality,cast(null as number) filtered_cardinality from DELPHIXDB.SUPPLIERS SUP2 union all select ' 12' id,'DELPHIXDB' table_owner,'ALTERNATIVES' table_name,'ALT' table_alias,17900 num_rows,count(*) rowcount,17900 cardinality,cast(null as number) filtered_cardinality from DELPHIXDB.ALTERNATIVES ALT union all select ' 19' id,'DELPHIXDB' table_owner,'ORDER_LINES' table_name,'ORL' table_alias,286416 num_rows,count(*) rowcount,286416 cardinality,cast(null as number) filtered_cardinality from DELPHIXDB.ORDER_LINES ORL union all select null,null,null,null,null,null,null,null from dual ) select frp_data.*,round(frp_data.filtered_cardinality/case when frp_data.rowcount = 0 then cast(null as number) else frp_data.rowcount end*100,1) actual_frp,decode(frp_data.filtered_cardinality,null,cast(null as number),round(frp_data.cardinality/case when frp_data.num_rows = 0 then cast(null as number) else frp_data.num_rows end*100,1)) plan_frp from frp_data where id is not null order by frp_data.id /
If I spool this to a file like kmo.sql (Kevin Meade out ) and run it I’ll get the filter ratios
@kmo.sql ID TABLE_OWN TABLE_NAME TABL NUM_ROWS ROWCOUNT CARDINALITY FILTERED_CARDINALITY ACTUAL_FRP PLAN_FRP ----- --------- ------------ ---- ---------- ---------- ----------- -------------------- ---------- -------- 2 DELPHIXDB SUPPLIERS SUP1 99 99 50 49 49.5 50.5 8 DELPHIXDB SUPPLIERS SUP2 99 99 50 50 50.5 50.5 11 DELPHIXDB PRODUCTS PRD2 1999 1999 1999 12 DELPHIXDB ALTERNATIVES ALT 17900 17900 17900 14 DELPHIXDB PRODUCTS PRD1 1999 1999 1999 17 DELPHIXDB CUSTOMERS CUS 14576 14576 49 49 .3 .3 18 DELPHIXDB ORDERS ORD 71604 71604 17898 8941 12.5 25.0 19 DELPHIXDB ORDER_LINES ORL 286416 286416 286416
- NUM_ROWS – rows from table statistics
- ROWCOUT – actual count(*)
- CARDINALITY – optimizer expected cardinality
- ACTUAL_FRP – actual filter ratio
- PLAN_FRP – expected filter ratio
From here I can see that the best filter ratio is on Customers and that’s where I should start my query.
A full set of scripts from his book are available here scripts.rar
A short word doc on tuning from Kevin along with some examples is available at information needed in Tuning a SQL Query.docx