Ok, since it seems that my last two questions (this one and this one) only lead to confussion, I will try to explain the FULL problem here, so it might be a long post.
I'm trying to create a database for a trading system. The database has 2 main tables. One is table "Ticks" and the other is "Candles". As shown in the figure, each table has its own attributes..
Candles, bars or ohlc are the same thing.
The way a candle is seen in a chart is like this:
Candles are just a way to representate aggregated data, nothing more.
There are many ways to aggregate ticks in order to create one candle. In this post, I'm asking for a particular way that is creating one candle every 500 ticks. So, if the ticks table has 1000 ticks, I can create only 2 candles. If it has 500 ticks, I can create 1 candle. If it has 5000 ticks, I can create 10 candles. If there are 5001 ticks I still have only 10 candles, because I'm missing the other 499 ticks in order to create the 11th candle.
Actually, I'm storing all the ticks using a python script and creating (and therefore, inserting in the candles table) candles with another python script. This is a real time process.
Both scripts run in a while True: loop. No, I can't (read shouldn't) stop the scripts because the market is opened 24 hours - 5 days a week.
What I'm trying to do is to get rid of the python script that creates and stores the candles in the candles table. Why? Because I think that it will improve performance. Instead of doing multiple queries to know the amount of ticks that are available to create a new candle, I think that a trigger could handle it in a more efficient way (please, if I'm mistaken correct me).
I don't know how to actually solve it, but what I'm trying is to do this (thanks to #GordonLinoff for helping me in previous questions):
do $$
begin
with total_ticks as (
select count(*) c from (
select * from eurusd_tick2 eurusd where date >
(SELECT date from eurusd_ohlc order by date desc limit 1)
order by date asc) totals),
ticks_for_candles as(
select * from eurusd_tick2 eurusd where date >
(SELECT date from eurusd_ohlc order by date desc limit 1)
order by date asc
), candles as(
select max(date) as date,
max(bid) filter (where mod(seqnum, 500) = 1) as open,
max(bid) as high,
min(bid) as low,
max(bid) filter (where mod(seqnum, 500) = 500-1) as close,
max(ask) filter (where mod(seqnum, 500) = 500-1) as ask
from (
select t.*, row_number() over (order by date) as seqnum
from (select * from ticks_for_candles) t) as a
group by floor((seqnum - 1) /500)
having count(*) = 500
)
case 500<(select * from total_ticks)
when true then
return select * from candles
end;
end $$;
Using this, I get this error:
ERROR: syntax error at or near "case"
LINE 33: case 500<(select * from total_ticks)
^
SQL state: 42601
Character: 945
As you can see, there is no select after the CETs. If I put:
select case 500<(select * from total_ticks)
when true then
return select * from candles
end;
end $$;
I get this error:
ERROR: subquery must return only one column
LINE 31: (select * from candles)
^
QUERY: with total_ticks as (
select count(*) c from (
select * from eurusd_tick2 eurusd where date >
(SELECT date from eurusd_ohlc order by date desc limit 1)
order by date asc) totals),
ticks_for_candles as(
select * from eurusd_tick2 eurusd where date >
(SELECT date from eurusd_ohlc order by date desc limit 1)
order by date asc
), candles as(
select max(date) as date,
max(bid) filter (where mod(seqnum, 500) = 1) as open,
max(bid) as high,
min(bid) as low,
max(bid) filter (where mod(seqnum, 500) = 500-1) as close,
max(ask) filter (where mod(seqnum, 500) = 500-1) as ask
from (
select t.*, row_number() over (order by date) as seqnum
from (select * from ticks_for_candles) t) as a
group by floor((seqnum - 1) /500)
having count(*) = 500
)
select case 1000>(select * from total_ticks)
when true then
(select * from candles)
end
CONTEXT: PL/pgSQL function inline_code_block line 4 at SQL statement
SQL state: 42601
So honestly, I don't know how to do it correctly. It doesn't has to be with the actual code I provide here, but the desired output looks as follows:
-----------------------------------------------------------------------------------
| date | open | high | low | close | ask |
|2020-05-01 20:39:27.603452| 1.0976 | 1.09766 | 1.09732 | 1.09762 | 1.09776 |
This would be the output when there is enough ticks to create only 1 candle. If there is enough to create two of them, then there should be 2 rows.
So, at the end of the day, what I have in mind is that the trigger should check constantly if there is enough data to create a candle and if it is, then create it.
Is this a good idea or I should stick to the python script?
Can this be achieved with my approach?
What I'm doing wrong?
What should I do and how should I manage this situation?
I really hope that the question now is complete and there is no missing information.
All comments and advices are appreciated.
Thanks!
EDIT: Since this is a real time process, in one second there could be 499 ticks in the database and in the next second there could be 503 ticks. This means that 4 ticks arrived in 1 second.
Being a database guy, my approach would be to use triggers in the database.
Create a third table candle_in_the_making that contains the data from the ticks that have not yet been aggregated to a candles entry.
Create an INSERT trigger on the ticks table (doesn't matter if BEFORE or AFTER) that does the following:
For every tick inserted, add a row to candle_in_the_making.
If the row count reaches 500, compute and insert a new candles row and TRUNCATE candle_in_the_making.
This is simple if ticks are inserted only in a single thread.
If ticks are inserted concurrently, you have to find a way to prevent two threads from inserting the 500th tick in candle_in_the_making at the same time (so that you end up with 501 entries). I can think of two ways to do that in the database:
Have an extra table c_i_m_count that contains only a single number, which is the number of rows in candle_in_the_making. Before you insert into candle_in_the_making, you run the atomic
UPDATE c_i_m_count SET counter = counter + 1 RETURNING counter;
This locks the row, so that any two INSERTs into counter_in_the_making are effectively serialized.
Use advisory locks to serialize the inserting threads. In particular, a transaction level exclusive lock as taken by pg_advisory_xact_lock would be indicated.
Related
I've been able to find a few examples of questions similar to this one, but most only involve a single column being checked.
SQL Select until Quantity Met
Select rows until condition met
I have a large table representing facilities, with columns for each type of resource available and the number of those specific resources available per facility. I want this stored procedure to be able to take integer values in as multiple parameters (representing each of these columns) and a Lat/Lon. Then it should iterate over the table sorted by distance, and return all rows (facilities) until the required quantity of available resources (specified by the parameters) are met.
Data source example:
Id
Lat
Long
Resource1
Resource2
...
1
50.123
4.23
5
12
...
2
61.234
5.34
0
9
...
3
50.634
4.67
21
18
...
Result Wanted:
#latQuery = 50.634
#LongQuery = 4.67
#res1Query = 10
#res2Query = 20
Id
Lat
Long
Resource1
Resource2
...
3
50.634
4.67
21
18
...
1
50.123
4.23
5
12
...
Result includes all rows that meet the queries individually. Result is also sorted by distance to the requested lat/lon
I'm able to sort the results by distance, and sum the total running values as suggested in other threads, but I'm having some trouble with the logic comparing the running values with the quota provided in the params.
First I have some CTEs to get most recent edits, order by distance and then sum the running totals
WITH cte1 AS (SELECT
#origin.STDistance(geography::Point(Facility.Lat, Facility.Long, 4326)) AS distance,
Facility.Resource1 as res1,
Facility.Resource2 as res2
-- ...etc
FROM Facility
),
cte2 AS (SELECT
distance,
res1,
SUM(res1) OVER (ORDER BY distance) AS totRes1,
res2,
SUM(res1) OVER (ORDER BY distance) AS totRes2
-- ...etc, there's 15-20 columns here
FROM cte1
)
Next, with the results of that CTE, I need to pull rows until all quotas are met. Having the issues here, where it works for one row but my logic with all the ANDs isn't exactly right.
SELECT * FROM cte2 WHERE (
(totRes1 <= #res1Query OR (totRes1 > #res1Query AND totRes1- res1 <= #totRes1)) AND
(totRes2 <= #res2Query OR (totRes2 > #res2Query AND totRes2- res2 <= #totRes2)) AND
-- ... I also feel like this method of pulling the next row once it's over may be convoluted as well?
)
As-is right now, it's mostly returning nothing, and I'm guessing it's because it's too strict? Essentially, I want to be able to let the total values go past the required values until they are all past the required values, and then return that list.
Has anyone come across a better method of searching using separate quotas for multiple columns?
See my update in the answers/comments
I think you are massively over-complicating this. This does not need any joins, just some running sum calculations, and the right OR logic.
The key to solving this is that you need all rows, where the running sum up to the previous row is less than the requirement for all requirements. This means that you include all rows where the requirement has not been met, and the first row for which the requirement has been met or exceeded.
To do this you can subtract the current row's value from the running sum.
You could utilize a ROWS specification of ROWS BETWEEN UNBOUNDED PRECEDING AND 1 PRECEDING. But then you need to deal with NULL on the first row.
In any event, even a regular running sum should always use ROWS UNBOUNDED PRECEDING, because the default is RANGE UNBOUNDED PRECEDING, which is subtly different and can cause incorrect results, as well as being slower.
You can also factor out the distance calculation into a CROSS APPLY (VALUES, avoiding the need for lots of CTEs or derived tables. You now only need one level of derivation.
DECLARE #origin geography = geography::Point(#latQuery, #LongQuery, 4326);
SELECT
f.Id,
f.Lat,
f.Long,
f.Resource1,
f.Resource2
FROM (
SELECT f.*,
SumRes1 = SUM(f.Resource1) OVER (ORDER BY v1.Distance ROWS UNBOUNDED PRECEDING) - f.Resource1,
SumRes2 = SUM(f.Resource2) OVER (ORDER BY v1.Distance ROWS UNBOUNDED PRECEDING) - f.Resource2
FROM Facility f
CROSS APPLY (VALUES(
#origin.STDistance(geography::Point(f.Lat, f.Long, 4326))
)) v1(Distance)
) f
WHERE (
f.SumRes1 < #res1Query
OR f.SumRes2 < #res2Query
);
db<>fiddle
Was able to figure out the problem on my own here. The primary issue I was running into was that I was comparing 25 different columns' running totals versus the 25 stored proc parameters (quotas of resources required by the search).
Changing the lines such as these
(totRes1 <= #res1Query OR (totRes1 > #res1Query AND totRes1- res1 <= #totRes1)) AND --...
to
(totRes1 <= #res1Query OR (totRes1 > #res1Query AND totRes1- res1 <= #totRes1) OR #res1Query = 0) AND --...
(adding in the OR #res1Query = 0)solved my issue.
In other words, the search is often only for one or two columns (types of resources) - leaving others as zero. The way my logic was set up caused it to skip over lots of rows because it was instantly marking them as having met the quota (value less than or equal to the quota). like #A Neon Tetra suggested, was pretty close to it already.
Update:
First attempt didn't exactly fix my own issue. Posting the stripped down version of my code that is now working for me.
DECLARE #Lat AS DECIMAL(12,6)
DECLARE #Lon AS DECIMAL(12,6)
DECLARE #res1Query AS INT
DECLARE #res2Query AS INT
-- repeat for Resource 3 through 25, etc...
DECLARE #origin geography = geography::Point(#Lat, #Lon, 4326);
-- CTE to be able to expose distance
cte AS (SELECT TOP(99999) -- --> this is hacky, it won't let me order by distance unless I'm selecting TOP(x) or some other fn?
dbo.Facility.FacilityGUID,
dbo.Facility.Lat,
dbo.Facility.Lon,
#origin.STDistance(geography::Point(dbo.Facility.Lat, dbo.Facility.Lon, 4326))
AS distance,
dbo.Facility.Resource1 AS res1,
dbo.Facility.Resource2 AS res2,
-- repeat for Resource 3 through 25, etc...
FROM dbo.Facility
ORDER BY distance),
-- third CTE - has access to distance so we can keep track of a running total ordered by distance
---> have to separate into two since you can't reference the same alias (distance) again within the same SELECT
fullCTE AS (SELECT
FacilityID,
Lat,
Long,
distance,
res1,
SUM(res1) OVER (ORDER BY distance)AS totRes1,
res2,
SUM(res2) OVER (ORDER BY distance)AS totRes2,
-- repeat for Resource 3 through 25, etc...
FROM cte)
SELECT * -- Customize what you're pulling here for your output as needed
FROM dbo.Facility INNER JOIN fullCTE ON (fullCTE.FacilityID = dbo.Facility.FacilityID)
WHERE EXISTS
(SELECT
FacilityID
FROM fullCTE WHERE (
FacilityID = dbo.Facility.FacilityID AND
-- Keep pulling rows until all conditions are met, as opposed to pulling rows while they're under the quota
NOT (
((totRes1 - res1 >= #res1Query AND #res1Query <> 0) OR (#res1Query = 0)) AND
((totRes2 - res2 >= #res2Query AND #res2Query <> 0) OR (#res2Query = 0)) AND
-- repeat for Resource 3 through 25, etc...
)
)
)
Please help me to optimize my SQL query.
I have a table with the fields: date, commodity_id, exp_month_id, exp_year, price, where the first 4 fields are the primary key. The months are designated with the alphabet-ordered letters: e.g. F (for Jan), G (for Feb.), H (for March), etc. Thus the letter of more distant from Jan. month will be larger than the letter of the less distant month (F < G < H < ...). Some commodity_ids have all 12 months in the table, some only 5 or 3, which are constant for all years.
I need to calculate the difference between prices (gradient) of the neighboring records in terms of exp_month_id, exp_year. As the first step, I want to define for every couple (exp_month_id, exp_year) the valid couple (next_month_id, next_year). The main problem here, that if the current exp_month_id is the last in the year, then next_year = exp_year + 1 and next_month_id should be the first one in the year.
I have written the following query to do the job:
WITH trading_months AS (
SELECT DISTINCT commodity_id,
exp_month_id
FROM futures
ORDER BY exp_month_id
)
SELECT DISTINCT f.commodity_id,
f.exp_month_id,
f.exp_year,
(
WITH [temp] AS (
SELECT exp_month_id
FROM trading_months
WHERE commodity_id = f.commodity_id
)
SELECT exp_month_id
FROM [temp]
WHERE exp_month_id > f.exp_month_id
UNION ALL
SELECT exp_month_id
FROM [temp]
LIMIT 1
)
AS next_month_id,
(
SELECT CASE WHEN EXISTS (
SELECT commodity_id,
exp_month_id
FROM trading_months
WHERE commodity_id = f.commodity_id AND
exp_month_id > f.exp_month_id
LIMIT 1
)
THEN f.exp_year ELSE f.exp_year + 1 END
)
AS next_year
FROM futures AS f
This query serves as a base for a dynamic table (view) which is subsequently used for calculating the gradient. However, the execution of this query takes more than one second and thus the whole process takes minutes. I wonder if you could help me optimizing the query.
Note: The following requires Sqlite 3.25 or newer for window function support:
Lack of sample data (Preferably as a CREATE TABLE and INSERT statements for easy importing) and expected results makes it hard to test, but if your end goal is computing the difference in prices between expiration dates (Making your question a bit of an XY problem, maybe something like:
SELECT date, commodity_id, price, exp_year, exp_month_id
, price - lag(price, 1) OVER (PARTITION BY commodity_id ORDER BY exp_year, exp_month_id) AS "change from last price"
FROM futures;
Thanks to the hint of #Shawn to use window functions I could rewrite the query in much shorter form:
CREATE VIEW "futures_nextmonths_win" AS
WITH trading_months AS (
SELECT DISTINCT commodity_id,
exp_month_id,
exp_year
FROM futures)
SELECT commodity_id,
exp_month_id,
exp_year,
lead(exp_month_id) OVER w AS next_month_id,
lead(exp_year) OVER w AS next_year
FROM trading_months
WINDOW w AS (PARTITION BY commodity_id ORDER BY exp_year, exp_month_id);
which is also slightly faster then the original one.
Suppose I have a database of athletic meeting results with a schema as follows
DATE,NAME,FINISH_POS
I wish to do a query to select all rows where an athlete has competed in at least three events without winning. For example with the following sample data
2013-06-22,Johnson,2
2013-06-21,Johnson,1
2013-06-20,Johnson,4
2013-06-19,Johnson,2
2013-06-18,Johnson,3
2013-06-17,Johnson,4
2013-06-16,Johnson,3
2013-06-15,Johnson,1
The following rows:
2013-06-20,Johnson,4
2013-06-19,Johnson,2
Would be matched. I have only managed to get started at the following stub:
select date,name FROM table WHERE ...;
I've been trying to wrap my head around the where clause but I can't even get a start
I think this can be even simpler / faster:
SELECT day, place, athlete
FROM (
SELECT *, min(place) OVER (PARTITION BY athlete
ORDER BY day
ROWS 3 PRECEDING) AS best
FROM t
) sub
WHERE best > 1
->SQLfiddle
Uses the aggregate function min() as window function to get the minimum place of the last three rows plus the current one.
The then trivial check for "no win" (best > 1) has to be done on the next query level since window functions are applied after the WHERE clause. So you need at least one CTE of sub-select for a condition on the result of a window function.
Details about window function calls in the manual here. In particular:
If frame_end is omitted it defaults to CURRENT ROW.
If place (finishing_pos) can be NULL, use this instead:
WHERE best IS DISTINCT FROM 1
min() ignores NULL values, but if all rows in the frame are NULL, the result is NULL.
Don't use type names and reserved words as identifiers, I substituted day for your date.
This assumes at most 1 competition per day, else you have to define how to deal with peers in the time line or use timestamp instead of date.
#Craig already mentioned the index to make this fast.
Here's an alternative formulation that does the work in two scans without subqueries:
SELECT
"date", athlete, place
FROM (
SELECT
"date",
place,
athlete,
1 <> ALL (array_agg(place) OVER w) AS include_row
FROM Table1
WINDOW w AS (PARTITION BY athlete ORDER BY "date" ASC ROWS BETWEEN 3 PRECEDING AND CURRENT ROW)
) AS history
WHERE include_row;
See: http://sqlfiddle.com/#!1/fa3a4/34
The logic here is pretty much a literal translation of the question. Get the last four placements - current and the previous 3 - and return any rows in which the athlete didn't finish first in any of them.
Because the window frame is the only place where the number of rows of history to consider is defined, you can parameterise this variant unlike my previous effort (obsolete, http://sqlfiddle.com/#!1/fa3a4/31), so it works for the last n for any n. It's also a lot more efficient than the last try.
I'd be really interested in the relative efficiency of this vs #Andomar's query when executed on a dataset of non-trivial size. They're pretty much exactly the same on this tiny dataset. An index on Table1(athlete, "date") would be required for this to perform optimally on a large data set.
; with CTE as
(
select row_number() over (partition by athlete order by date) rn
, *
from Table1
)
select *
from CTE cur
where not exists
(
select *
from CTE prev
where prev.place = 1
and prev.athlete = cur.athlete
and prev.rn between cur.rn - 3 and cur.rn
)
Live example at SQL Fiddle.
A short description: I have a table with data that is updated over a certain time period. Now the problem is, that - depending on the nature of the sensor which sends the data - in this time period there could be either 50 data sets or 50.000. As I want to visualize this data (using ASP.NET / c#), for a first preview I would like to SELECT just 1000 values from the table.
I already have an approach doing this: I count the rows in the time period of interest, with a simple "where" clause to specify the sensor-id, save it as a variable in SQL, and then divide the count() by 1000. I've tried it in MS Access, where it works just fine:
set #divider = select count(*) from table where [...]
SELECT (Int([RowNumber]/#divider)), First(Value)
FROM myTable
GROUP BY (Int([RowNumber]/#divider));
The trick in Access was, that I simply have a data field ("RowNumber"), which is my PK/ID, and goes from 0 up. I tried to accomplish that in SQL Server using the ROW_NUMBER() method, which works more or less. I've got the right syntax for the method, but I can not use the GROUP BY statement
Windowed functions can only appear in the SELECT or ORDER BY
clauses.
meaning ROW_NUMBER() can't be in the GROUP BY statement.
Now I'm kinda stuck. I've tried to save the ROW_NUMBER value into a char or a separate column, and GROUP BY it later on, but I couldn't get it done. And somehow I start to think, that my strategy might have its weaknesses ...? :/
To clarify once more: I don't need to SELECT TOP 1000 from my table, because this would just mean that I select the first 1000 values (depending on the sorting). I need to SELECT every x-th value, while I can compute the x (and I could even round it to an INT, if that would help to get it done). I hope I was able to describe the problem understandable ...
This is my first post here on StackOverflow, I hope I didn't forget anything essential or important, if you need any further information (table structure, my queries so far, ...) please don't hesitate to ask. Any help or hint is highly appreciated - thanks in advance! :)
Update: SOLUTION! Big thanks to https://stackoverflow.com/users/52598/lieven!!!
Here is how I did it in the end:
I declare 2 variables - I count my rows and SET it into the first var. Then I use ROUND() on the just assigned variable, and divide it by 1000 (because in the end I want ABOUT 1000 values!). I split this operation into 2 variables, because if I used the value from the COUNT function as basis for my ROUND operation, there were some mistakes.
declare #myvar decimal(10,2)
declare #myvar2 decimal(10,2)
set #myvar = (select COUNT(*)
from value_table
where channelid=135 and myDate >= '2011-01-14 22:00:00.000' and myDate <= '2011-02-14 22:00:00.000'
)
set #myvar2 = ROUND(#myvar/1000, 0)
Now I have the rounded value, which I want to be my step-size (take every x-th value -> this is our "x" ;)) stored in #myvar2. Next I will subselect the data of the desired timespan and channel, and add the ROW_NUMBER() as column "rn", and finally add a WHERE-clause to the outer SELECT, where I divide the ROW_NUMBER through #myvar2 - when the modulus is 0, the row will be SELECTed.
select * from
(
select (ROW_NUMBER() over (order by id desc)) as rn, myValue, myDate
from value_table
where channel_id=135 and myDate >= '2011-01-14 22:00:00.000' and myDate<= '2011-02-14 22:00:00.000'
) d
WHERE rn % #myvar2 = 0
Works like a charm - once again all my thanks to https://stackoverflow.com/users/52598/lieven, see the comment below for the original posting!
In essence, all you need to do to select the x-th value is retain all rows where the modulus of the rownumber divided by x is 0.
WHERE rn % #x_thValues = 0
Now to be able to use your ROW_NUMBER's result, you'll need to wrap the entire statement into in a subselect
SELECT *
FROM (
SELECT *
, rn = ROW_NUMBER() OVER (ORDER BY Value)
FROM DummyData
) d
WHERE rn % #x_thValues = 0
Combined with a variable to what x-th values you need, you might use something like this testscript
DECLARE #x_thValues INTEGER = 2
;WITH DummyData AS (SELECT * FROM (VALUES (1), (2), (3), (4)) v (Value))
SELECT *
FROM (
SELECT *
, rn = ROW_NUMBER() OVER (ORDER BY Value)
FROM DummyData
) d
WHERE rn % #x_thValues = 0
One more option to consider:
Select Top 1000 *
From dbo.SomeTable
Where ....
Order By NewID()
but to be honest- like the previous answer more than this one.
The question could be about performance..
I have a MySQL table with approximately 3000 rows per user. One of the columns is a datetime field, which is mutable, so the rows aren't in chronological order.
I'd like to visualize the time distribution in a chart, so I need a number of individual datapoints. 20 datapoints would be enough.
I could do this:
select timefield from entries where uid = ? order by timefield;
and look at every 150th row.
Or I could do 20 separate queries and use limit 1 and offset.
But there must be a more efficient solution...
Michal Sznajder almost had it, but you can't use column aliases in a WHERE clause in SQL. So you have to wrap it as a derived table. I tried this and it returns 20 rows:
SELECT * FROM (
SELECT #rownum:=#rownum+1 AS rownum, e.*
FROM (SELECT #rownum := 0) r, entries e) AS e2
WHERE uid = ? AND rownum % 150 = 0;
Something like this came to my mind
select #rownum:=#rownum+1 rownum, entries.*
from (select #rownum:=0) r, entries
where uid = ? and rownum % 150 = 0
I don't have MySQL at my hand but maybe this will help ...
As far as visualization, I know this is not the periodic sampling you are talking about, but I would look at all the rows for a user and choose an interval bucket, SUM within the buckets and show on a bar graph or similar. This would show a real "distribution", since many occurrences within a time frame may be significant.
SELECT DATEADD(day, DATEDIFF(day, 0, timefield), 0) AS bucket -- choose an appropriate granularity (days used here)
,COUNT(*)
FROM entries
WHERE uid = ?
GROUP BY DATEADD(day, DATEDIFF(day, 0, timefield), 0)
ORDER BY DATEADD(day, DATEDIFF(day, 0, timefield), 0)
Or if you don't like the way you have to repeat yourself - or if you are playing with different buckets and want to analyze across many users in 3-D (measure in Z against x, y uid, bucket):
SELECT uid
,bucket
,COUNT(*) AS measure
FROM (
SELECT uid
,DATEADD(day, DATEDIFF(day, 0, timefield), 0) AS bucket
FROM entries
) AS buckets
GROUP BY uid
,bucket
ORDER BY uid
,bucket
If I wanted to plot in 3-D, I would probably determine a way to order users according to some meaningful overall metric for the user.
#Michal
For whatever reason, your example only works when the where #recnum uses a less than operator. I think when the where filters out a row, the rownum doesn't get incremented, and it can't match anything else.
If the original table has an auto incremented id column, and rows were inserted in chronological order, then this should work:
select timefield from entries
where uid = ? and id % 150 = 0 order by timefield;
Of course that doesn't work if there is no correlation between the id and the timefield, unless you don't actually care about getting evenly spaced timefields, just 20 random ones.
Do you really care about the individual data points? Or will using the statistical aggregate functions on the day number instead suffice to tell you what you wish to know?
AVG
STDDEV_POP
VARIANCE
TO_DAYS
select timefield
from entries
where rand() = .01 --will return 1% of rows adjust as needed.
Not a mysql expert so I'm not sure how rand() operates in this environment.
For my reference - and for those using postgres - Postgres 9.4 will have ordered set aggregates that should solve this problem:
SELECT percentile_disc(0.95)
WITHIN GROUP (ORDER BY response_time)
FROM pageviews;
Source: http://www.craigkerstiens.com/2014/02/02/Examining-PostgreSQL-9.4/