Postgres seems pretty cool as open source databases go. I'm finding a much greater affinity for Postgres than say MySQL.
On the other hand, coming from an Oracle background there are a few things that come across as strange on Postgres. Most of the weirdness for me centers around MVCC,
Here is one new surprise for me.
I'm running a steady workload on a set of tables with inserts, updates, deletes and selects. This is putting a decent load on the database.
Then at 20:45, I simply do:
\set AUTOCOMMIT off
\echo :AUTOCOMMIT
create table toto ( id int);
insert into toto values (1);
commit;
select * from toto for ...
khailey
Uncategorized
Covering indexes are where all the columns needed are in the index and, in theory, only the index needs to be accessed. This seemingly straight forward strategy is much tricker on Postgres.
Let's take TPC-H query 6:
select
sum(l_extendedprice * l_discount) as revenue
from
lineitem
where
l_shipdate >= date '1996-01-01'
and l_shipdate < cast(date '1996-01-01' + interval '1 year' as date)
and l_discount between 0.06 - ...
khailey
Uncategorized
The longer the timeline we show in the graph, either the more points we show or the larger granularity we use.
For example if I show 1 hour of data, then I can show 60 points of 1 minute each granularity.
If I showed a day at 1 minute each then that would be 1440 points.
Say we wanted to limit the points to less than 300 points so our UI stays responsive no matter the time scale.
The for 24 hours I can show 5 minute granularity which is 288 points
If I show 25 hours that's 300 points at 5 minute granularity so we ...
khailey
Uncategorized
Summary
Goal in this post is to layout a methodology that can be implemented to scan the explain plans and identify opportunities to optimize the sql execution with additional indexes.
The missing index opportunities outlined here are
Indexes that would avoid full table scan with a predicate filter that filters out most of the returned rows
Joins that are missing index on the join field
Index range scans that could be made more efficient by addition of a field to existing index
Also for future consideration, but not yet outline here
Recommending covering index where the addition of fields to an index avoids table access all together
Indexes that ...
khailey
Uncategorized
MySQL is simpler than the Oracle example because MySQL only has Nested Loops and doesn't have Hash Join.
Here is the query, like on the Oracle example from the last blog post
select max(t1.data)
from t1, t2
where t1.id = t2.id
and t1.clus = 1
;
So there are only 4 examples below which all do NL joins
full table scan on T1 and T2
7.83 secs
index on T1 predicate filter column
7.39 secs
index on T2 join column
0.49 secs
index on both T2 join column and T1 predicate filter column
0.06 secs
There isn't an idea of "explain analyze" on MySQL until MySQL 8.0.18 and I did my ...
khailey
Uncategorized
Looking at ways to programmatically analyze explain plans to see if adding indexes would help.
The analysis is straight forward for for full table scans with strong predicate filters. If there is a strong predicate filter on an explain plan node with a full table scan then it's a good candidate for an index.
Now a similar situation exists for a join.
Again if the join has a strong predicate filter on a full table scan, that is a candidate.
On the other hand when the actual join fields look like they will benefit from an index, it's less obvious.
In the queries below T1 and T2 have the ...
khailey
Uncategorized
Had and old export dump file I wanted to import into RDS.
I first tried to use S3 as a hosting site for the .dmp file and use rdsadmin.rdsadmin_s3_tasks.download_from_s3 to copy the file to the RDS database then import it with DBMS_DATAPUMP.
Unfortunately DBMS_DATAPUMP only works with expdp files so instead of using DBMS_DATAPUMP , I got the old "imp" binary and used it with TWO_TASK to the RDS database I wanted it loaded into.
For the record here were the steps I took
First put the file into an S3 bucket called
kylelf-datadump
Create an IAM policy to allow Amazon RDS access to an Amazon S3 bucket
https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/oracle-s3-integration.html#oracle-s3-integration.preparing
My policy ...
khailey
Uncategorized
How does one check if there are missing indexes in a database?
Is there a programatic way to do it?
Explain Plan
Most blogs on the subject will say look at the explain plan for your query.
Basic concept is to look for full table scans in the explain plan and the look and see if there is a predicate filter (i.e. SQL text of form “where table.field=value”) with a full table scan or a full table scan in a join ( i.e. tableA.field = tableB.field). There are other considerations, things to check, and other places for indexes, but this is a good start.
Explain ...
khailey
Uncategorized
trying to connect to SQL Server from my Mac so install sqlcmd:
brew tap microsoft/mssql-release https://github.com/Microsoft/homebrew-mssql-release
brew update
brew install mssql-tools
then get error
$ sqlcmd -S kylelfsqls.xxxxxx.us-west-2.amazonaws.com,1433 -U kylelf
Sqlcmd: Error: Microsoft ODBC Driver 17 for SQL Server : SSL Provider: [OpenSSL library could not be loaded, make sure OpenSSL 1.0 or 1.1 is installed].
Sqlcmd: Error: Microsoft ODBC Driver 17 for SQL Server : Client unable to establish connection.
Segmentation fault: 11
Did't find much info google, so blogging here.
Ran
ln -s /usr/local/opt/openssl@1.1 /usr/local/opt/openssl
and it worked
khailey
Uncategorized
Goal was to create a empty table and poplulate it with optimizer table and index stats from a poplulated table to try and get the optimizer to give the same explain plan and costs on the empty table as the populated table. The purpose of this was to be able to modify the empty table by adding indexes quickly and cheaply and still be able to get correct optimizer costs. This worked in the case of table access but unfortunately didn't work when bring in an index.
Create procedure to populate test table
DROP PROCEDURE IF EXISTS populate_data;
DELIMITER //
CREATE PROCEDURE populate_data(max_val int)
BEGIN
DECLARE ...
khailey
Uncategorized
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