I'm querying Clickhouse with a query that has ORDER BY and LIMIT 1, and the ORDER BY matches the table's sort order. The query returns 1 row as expected, however, 50+ rows were scanned to return the result.
I would expect ClickHouse to scan only 1 row as the ORDER BY is in the table's sort order. What's happening here and what can I do to fix this?
SELECT * FROM comp_intel_scrapes
order by
client_slug,
client_hotel_id,
argset_id,
scrape_datetime,
preferred_country,
preferred_currency,
adults,
children,
nights,
min_checkin_date,
max_checkin_date
limit 1
----
Elapsed: 0.004s
Read: 54 rows (8.84KB)
By the way, Clickhouse.com's cloud is being used here.
It depends on a table engine.
Primary index is sparse https://clickhouse.com/docs/en/guides/improving-query-performance/sparse-primary-indexes/sparse-primary-indexes-design/
Because of this CH is unable to read less than one granule ~8192 rows.
In my power Bi I would like to count rows for all my tables and having this output:
Table Name
Row count
Table1
126
Table2
985
Table3
998
...
...
As long as I have few tables I can do
NEWTABLE = UNION(
ROW("TableName","Table1", "Rowcount",ROWSCOUNT(Table1)),
ROW("TableName","Table2", "Rowcount",ROWSCOUNT(Table2)),
...
)
But this starts to be complicated when I have many tables.
Is there a way I can do it? Like a loop or something?
Thank you
If you only need a metrics then you can use DaxStudio -> ViewMetrics
where cardinality is your "rowCounts"
If you need something more, then you can get all table name from DMV
select * from $SYSTEM.TMSCHEMA_TABLES
populate this as another table in your model, and use M language to loop through.
here useful example:
https://community.powerbi.com/t5/Power-Query/Power-query-Counting-rows-from-all-table-in-query-editor-but-not/td-p/1198489
I have a table with about 1.5 million rows and three columns. Column 'timestamp' is of type REAL and indexed. I am accessing the SQLite database via PHP PDO.
The following three selects run in less than a millisecond:
select timestamp from trades
select timestamp + 1 from trades
select max(timestamp) from trades
The following select needs almost half a second:
select max(timestamp) + 1 from trades
Why is that?
EDIT:
Lasse has asked for a "explain query plan", I have run this within a PHP PDO query since I have no direct SQLite3 command line tool access at the moment. I guess it does not matter, here is the result:
explain query plan select max(timestamp) + 1 from trades:
[selectid] => 0
[order] => 0
[from] => 0
[detail] => SCAN TABLE trades (~1000000 rows)
explain query plan select max(timestamp) from trades:
[selectid] => 0
[order] => 0
[from] => 0
[detail] => SEARCH TABLE trades USING COVERING INDEX tradesTimestampIdx (~1 rows)
The reason this query
select max(timestamp) + 1 from trades
takes so long is that the query engine must, for each record, compute the MAX value and then add one to it. Computing the MAX value involves doing a full table scan, and this must be repeated for each record because you are adding one to the value.
In the query
select timestamp + 1 from trades
you are doing a calculation for each record, but the engine only needs to scan the entire table once. And in this query
select max(timestamp) from trades
the engine does have to scan the entire table, however it also does so only once.
From the SQLite documentation:
Queries that contain a single MIN() or MAX() aggregate function whose argument is the left-most column of an index might be satisfied by doing a single index lookup rather than by scanning the entire table.
I emphasized might from the documentation, because it appears that a full table scan may be necessary for a query of the form SELECT MAX(x)+1 FROM table
if column x be not the left-most column of an index.
I have 2 tables-
bpm_agent_data - 40 Million records , 5 Columns
bpm_loan_data - 20 Million records, 5 Columns
Now I ran a query in Hive-
select count(bpm_agent_data.AgentID), count(bpm_loan_data.LoanNumber) from bpm_agent_data JOIN bpm_loan_data where bpm_loan_data.id = bpm_agent_data.id;
which is taking long long time to complete.
What should be the ideal way to write the query in HIVE so that Reducer must not take so much time.
Found the solution for the above query,
replaced where with ON
select count(bpm_agent_data.AgentID), count(bpm_loan_data.LoanNumber) from bpm_agent_data JOIN bpm_loan_data ON( bpm_loan_data.id = bpm_agent_data.id);
Have the following tables (Oracle 10g):
catalog (
id NUMBER PRIMARY KEY,
name VARCHAR2(255),
owner NUMBER,
root NUMBER REFERENCES catalog(id)
...
)
university (
id NUMBER PRIMARY KEY,
...
)
securitygroup (
id NUMBER PRIMARY KEY
...
)
catalog_securitygroup (
catalog REFERENCES catalog(id),
securitygroup REFERENCES securitygroup(id)
)
catalog_university (
catalog REFERENCES catalog(id),
university REFERENCES university(id)
)
Catalog: 500 000 rows, catalog_university: 500 000, catalog_securitygroup: 1 500 000.
I need to select any 50 rows from catalog with specified root ordered by name for current university and current securitygroup. There is a query:
SELECT ccc.* FROM (
SELECT cc.*, ROWNUM AS n FROM (
SELECT c.id, c.name, c.owner
FROM catalog c, catalog_securitygroup cs, catalog_university cu
WHERE c.root = 100
AND cs.catalog = c.id
AND cs.securitygroup = 200
AND cu.catalog = c.id
AND cu.university = 300
ORDER BY name
) cc
) ccc WHERE ccc.n > 0 AND ccc.n <= 50;
Where 100 - some catalog, 200 - some securitygroup, 300 - some university. This query return 50 rows from ~ 170 000 in 3 minutes.
But next query return this rows in 2 sec:
SELECT ccc.* FROM (
SELECT cc.*, ROWNUM AS n FROM (
SELECT c.id, c.name, c.owner
FROM catalog c
WHERE c.root = 100
ORDER BY name
) cc
) ccc WHERE ccc.n > 0 AND ccc.n <= 50;
I build next indexes: (catalog.id, catalog.name, catalog.owner), (catalog_securitygroup.catalog, catalog_securitygroup.index), (catalog_university.catalog, catalog_university.university).
Plan for first query (using PLSQL Developer):
http://habreffect.ru/66c/f25faa5f8/plan2.jpg
Plan for second query:
http://habreffect.ru/f91/86e780cc7/plan1.jpg
What are the ways to optimize the query I have?
The indexes that can be useful and should be considered deal with
WHERE c.root = 100
AND cs.catalog = c.id
AND cs.securitygroup = 200
AND cu.catalog = c.id
AND cu.university = 300
So the following fields can be interesting for indexes
c: id, root
cs: catalog, securitygroup
cu: catalog, university
So, try creating
(catalog_securitygroup.catalog, catalog_securitygroup.securitygroup)
and
(catalog_university.catalog, catalog_university.university)
EDIT:
I missed the ORDER BY - these fields should also be considered, so
(catalog.name, catalog.id)
might be beneficial (or some other composite index that could be used for sorting and the conditions - possibly (catalog.root, catalog.name, catalog.id))
EDIT2
Although another question is accepted I'll provide some more food for thought.
I have created some test data and run some benchmarks.
The test cases are minimal in terms of record width (in catalog_securitygroup and catalog_university the primary keys are (catalog, securitygroup) and (catalog, university)). Here is the number of records per table:
test=# SELECT (SELECT COUNT(*) FROM catalog), (SELECT COUNT(*) FROM catalog_securitygroup), (SELECT COUNT(*) FROM catalog_university);
?column? | ?column? | ?column?
----------+----------+----------
500000 | 1497501 | 500000
(1 row)
Database is postgres 8.4, default ubuntu install, hardware i5, 4GRAM
First I rewrote the query to
SELECT c.id, c.name, c.owner
FROM catalog c, catalog_securitygroup cs, catalog_university cu
WHERE c.root < 50
AND cs.catalog = c.id
AND cu.catalog = c.id
AND cs.securitygroup < 200
AND cu.university < 200
ORDER BY c.name
LIMIT 50 OFFSET 100
note: the conditions are turned into less then to maintain comparable number of intermediate rows (the above query would return 198,801 rows without the LIMIT clause)
If run as above, without any extra indexes (save for PKs and foreign keys) it runs in 556 ms on a cold database (this is actually indication that I oversimplified the sample data somehow - I would be happier if I had 2-4s here without resorting to less then operators)
This bring me to my point - any straight query that only joins and filters (certain number of tables) and returns only a certain number of the records should run under 1s on any decent database without need to use cursors or to denormalize data (one of these days I'll have to write a post on that).
Furthermore, if a query is returning only 50 rows and does simple equality joins and restrictive equality conditions it should run even much faster.
Now let's see if I add some indexes, the biggest potential in queries like this is usually the sort order, so let me try that:
CREATE INDEX test1 ON catalog (name, id);
This makes execution time on the query - 22ms on a cold database.
And that's the point - if you are trying to get only a page of data, you should only get a page of data and execution times of queries such as this on normalized data with proper indexes should take less then 100ms on decent hardware.
I hope I didn't oversimplify the case to the point of no comparison (as I stated before some simplification is present as I don't know the cardinality of relationships between catalog and the many-to-many tables).
So, the conclusion is
if I were you I would not stop tweaking indexes (and the SQL) until I get the performance of the query to go below 200ms as rule of the thumb.
only if I would find an objective explanation why it can't go below such value I would resort to denormalisation and/or cursors, etc...
First I assume that your University and SecurityGroup tables are rather small. You posted the size of the large tables but it's really the other sizes that are part of the problem
Your problem is from the fact that you can't join the smallest tables first. Your join order should be from small to large. But because your mapping tables don't include a securitygroup-to-university table, you can't join the smallest ones first. So you wind up starting with one or the other, to a big table, to another big table and then with that large intermediate result you have to go to a small table.
If you always have current_univ and current_secgrp and root as inputs you want to use them to filter as soon as possible. The only way to do that is to change your schema some. In fact, you can leave the existing tables in place if you have to but you'll be adding to the space with this suggestion.
You've normalized the data very well. That's great for speed of update... not so great for querying. We denormalize to speed querying (that's the whole reason for datawarehouses (ok that and history)). Build a single mapping table with the following columns.
Univ_id, SecGrp_ID, Root, catalog_id. Make it an index organized table of the first 3 columns as pk.
Now when you query that index with all three PK values, you'll finish that index scan with a complete list of allowable catalog Id, now it's just a single join to the cat table to get the cat item details and you're off an running.
The Oracle cost-based optimizer makes use of all the information that it has to decide what the best access paths are for the data and what the least costly methods are for getting that data. So below are some random points related to your question.
The first three tables that you've listed all have primary keys. Do the other tables (catalog_university and catalog_securitygroup) also have primary keys on them?? A primary key defines a column or set of columns that are non-null and unique and are very important in a relational database.
Oracle generally enforces a primary key by generating a unique index on the given columns. The Oracle optimizer is more likely to make use of a unique index if it available as it is more likely to be more selective.
If possible an index that contains unique values should be defined as unique (CREATE UNIQUE INDEX...) and this will provide the optimizer with more information.
The additional indexes that you have provided are no more selective than the existing indexes. For example, the index on (catalog.id, catalog.name, catalog.owner) is unique but is less useful than the existing primary key index on (catalog.id). If a query is written to select on the catalog.name column, it is possible to do and index skip scan but this starts being costly (and most not even be possible in this case).
Since you are trying to select based in the catalog.root column, it might be worth adding an index on that column. This would mean that it could quickly find the relevant rows from the catalog table. The timing for the second query could be a bit misleading. It might be taking 2 seconds to find 50 matching rows from catalog, but these could easily be the first 50 rows from the catalog table..... finding 50 that match all your conditions might take longer, and not just because you need to join to other tables to get them. I would always use create table as select without restricting on rownum when trying to performance tune. With a complex query I would generally care about how long it take to get all the rows back... and a simple select with rownum can be misleading
Everything about Oracle performance tuning is about providing the optimizer enough information and the right tools (indexes, constraints, etc) to do its job properly. For this reason it's important to get optimizer statistics using something like DBMS_STATS.GATHER_TABLE_STATS(). Indexes should have stats gathered automatically in Oracle 10g or later.
Somehow this grew into quite a long answer about the Oracle optimizer. Hopefully some of it answers your question. Here is a summary of what is said above:
Give the optimizer as much information as possible, e.g if index is unique then declare it as such.
Add indexes on your access paths
Find the correct times for queries without limiting by rowwnum. It will always be quicker to find the first 50 M&Ms in a jar than finding the first 50 red M&Ms
Gather optimizer stats
Add unique/primary keys on all tables where they exist.
The use of rownum is wrong and causes all the rows to be processed. It will process all the rows, assigned them all a row number, and then find those between 0 and 50. When you want to look for in the explain plan is COUNT STOPKEY rather than just count
The query below should be an improvement as it will only get the first 50 rows... but there is still the issue of the joins to look at too:
SELECT ccc.* FROM (
SELECT cc.*, ROWNUM AS n FROM (
SELECT c.id, c.name, c.owner
FROM catalog c
WHERE c.root = 100
ORDER BY name
) cc
where rownum <= 50
) ccc WHERE ccc.n > 0 AND ccc.n <= 50;
Also, assuming this for a web page or something similar, maybe there is a better way to handle this than just running the query again to get the data for the next page.
try to declare a cursor. I dont know oracle, but in SqlServer would look like this:
declare #result
table (
id numeric,
name varchar(255)
);
declare __dyn_select_cursor cursor LOCAL SCROLL DYNAMIC for
--Select
select distinct
c.id, c.name
From [catalog] c
inner join university u
on u.catalog = c.id
and u.university = 300
inner join catalog_securitygroup s
on s.catalog = c.id
and s.securitygroup = 200
Where
c.root = 100
Order by name
--Cursor
declare #id numeric;
declare #name varchar(255);
open __dyn_select_cursor;
fetch relative 1 from __dyn_select_cursor into #id,#name declare #maxrowscount int
set #maxrowscount = 50
while (##fetch_status = 0 and #maxrowscount <> 0)
begin
insert into #result values (#id, #name);
set #maxrowscount = #maxrowscount - 1;
fetch next from __dyn_select_cursor into #id, #name;
end
close __dyn_select_cursor;
deallocate __dyn_select_cursor;
--Select temp, final result
select
id,
name
from #result;