Wed Feb 01 02:37:46 GMT 2023
From
/weblog/database/performance
Note of this book -
http://www.ocelot.ca/tuning.htm 1) Order of "and" and "or" is matter, if no cost-base optimizer
where column1 = 'a' and column2 = 'b' -> where column2 = 'b' and column1 = 'a' if column2 'b' is less likely
2) <> is more expensive than =
3) Can force to use index if SELECT * FROM table WHERE indexed_column > 0
4) UPPER(column) might lose information, use LOWER is better
5) WHERE column = 'WORD' or column = 'word' is always faster than WHERE LOWER(column) = 'word' , even faster version is test if database if case-insensitive: WHERE column = 'WORD' or ('WORD' <> 'word' AND column = 'word')
6) 32bit integer is the fastest datatype for 32bit OS
7) column in (?, ?) is faster than column =? or column = ? for some database, and don't slow in any database
8) For aggregate functions, prevent using having, try to use fewer columns in group by and use subquery if possible
9) In fact, count(1) is not faster than count(*)
10) CAST before aggregate function: SELECT CAST(SUM(column) AS INT) -> SELECT SUM(CAST(column as INT))
11) Use constant as much as possible, e.g.: select * from t1, t2 where t1.c1 = t2.c1 and t1.c1 = 1 -> select * from t1, t2 where t1.c1=1 andn t2.c1=1
12) Information of Join vs Subqueries
13) Information of optimum column type (VARCHAR, TIMESTAMP, WORD size INT and CHAR(1) usually better), shift effect for page
14) prevent NULL for porting issue
15) Low level table physical information
Some related links -
http://www.techartifact.com[..]9/12/sql-optimization-tipsquestions.html 16) Prevent database level transaction, prevent joining, prevent locking ( like auto increment key ), pretty good suggestion -
http://www.aviransplace.com/2015/08/12/mysql-is-a-great-nosql/ 17) Reduce dataset for aggregation -
https://www.periscopedata.com[..]-subqueries-to-count-distinct-50x-faster Few good tips, like avoid cursors -
http://www.javaworld.com[..]ent/21-rules-for-faster-sql-queries.html 7 Tips for Query Optimization in SQL Servers -
https://learningdaily.dev[..]optimization-in-sql-servers-fb3653c910f1
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Thu Jun 24 14:57:56 GMT 2021
From
/weblog/database/performance
Case study about how to find out suitable index -
http://samsaffron.com[..]e+life+of+a+slow+page+at+Stack+Overflow# A simple example of how to use EXPLAIN to find out the bottleneck of the query and add suitable index to suitable column -
http://hackmysql.com/case4 Bitmap Index vs. B-tree Index: Which and When? -
http://www.oracle.com/technology/pub/articles/sharma_indexes.html http://publib.boulder.ibm.com[..]/v5r3/index.jsp?topic=/rzajq/perf21c.htm script to check if index is missing at foreign key
select table_name, constraint_name,
cname1 || nvl2(cname2,','||cname2,null) ||
nvl2(cname3,','||cname3,null) || nvl2(cname4,','||cname4,null) ||
nvl2(cname5,','||cname5,null) || nvl2(cname6,','||cname6,null) ||
nvl2(cname7,','||cname7,null) || nvl2(cname8,','||cname8,null)
columns
from ( select b.table_name,
b.constraint_name,
max(decode( position, 1, column_name, null )) cname1,
max(decode( position, 2, column_name, null )) cname2,
max(decode( position, 3, column_name, null )) cname3,
max(decode( position, 4, column_name, null )) cname4,
max(decode( position, 5, column_name, null )) cname5,
max(decode( position, 6, column_name, null )) cname6,
max(decode( position, 7, column_name, null )) cname7,
max(decode( position, 8, column_name, null )) cname8,
count(*) col_cnt
from (select substr(table_name,1,30) table_name,
substr(constraint_name,1,30) constraint_name,
substr(column_name,1,30) column_name,
position
from sys.user_cons_columns ) a,
sys.user_constraints b
where a.constraint_name = b.constraint_name
and b.constraint_type = 'R'
group by b.table_name, b.constraint_name
) cons
where col_cnt > ALL
( select count(*)
from sys.user_ind_columns i
where i.table_name = cons.table_name
and i.column_name in (cname1, cname2, cname3, cname4,
cname5, cname6, cname7, cname8 )
and i.column_position <= cons.col_cnt
group by i.index_name
)
http://chriswongdevblog.blogspot.com.au[..]01/dont-forget-to-index-your-oracle.html Explain how postgres indexes work, look like combine index is not that useful -
http://dublintech.blogspot.com.au/2015/07/postgres-indexes.html SQL Server JDBC – Set sendStringParametersAsUnicode to false -
https://vladmihalcea.com[..]rver-jdbc-sendstringparametersasunicode/
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Thu Jun 30 16:34:29 GMT 2011
From
/weblog/database/performance
When the query plan is compiled on first run of the procedure after a SQL Server restart, the query is optimised for those initial variables in that initial call.
There’s ways to write the procedure to have a more representative query plan chosen on initial compile each time:
http://blogs.msdn.com[..]ryoptteam/archive/2006/03/31/565991.aspx This is what is known as parameter sniffing.
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Fri Apr 29 01:34:07 GMT 2011
From
/weblog/database/performance
Tip of having faster 'select count(*) from table' of sql server, probably can use this as reference to check similar feature on other DB, if that hurt something -
http://beyondrelational.com[..]are-there-any-alternatives-to-count.aspx
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Fri Mar 27 03:49:44 GMT 2009
From
/weblog/database/performance
Oracle can help us to denormalize tables according to query via materialized view, should help performance a lot, probably there is similar feature in other DBMS -
http://www.oracle.com[..]ology/products/oracle9i/daily/jul05.html
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