Is there any easy way to get the number of rows returned by a sqlite statement? I don't want to have to go through the process of doing a COUNT() first. Thanks.
On each call to sqlite_step, increment a variable by 1.
If you want the row count in advance, then there's no easy way.
To count all entries in a table, you can use the following SQL statement:
SELECT COUNT(*) FROM "mytable" where something=42;
Or just the following to get all entries:
SELECT COUNT(*) FROM "mytable";
In case you have already done the query, and just want the number of entries returned you can use sqlite3_data_count() and sqlite3_column_count() depending on what you want to count.
Related
I use postgresql and I have a database table with more than 5 million records. The structure of the table is as follows:
A lot of records is inserted every day. There are many records with the same reference.
I want to select all records but I do not want duplicates, the records with the same reference.
I tried with query as follows:
SELECT DISTINCT ON (reference) reference_url, reference FROM daily_run_vehicle WHERE handled = False and retries < 5 ORDER BY reference DESC;
It executed, it gives me correct result, but it takes to long to execute.
Is there any better way to do this?
Create Sort keys on columns which yo used in where condition
after large data movement into the table, we need to do "vacuum" command it will refresh all the keys and after that Analyze the table with "Analyze" command. it will help to rebuild the stats of the table.
I know count table tells you how many rows are in table but how do you count from a table with a where clause as a filter? I tried count table where PERIOD=x but I am getting the error: 'PERIOD even though PERIOD is a field in the table
Use qsql to filter and then count the result:
count select from table where PERIOD=x
If you only need the count, do
exec sum PERIOD=x from table
If the table has many columns, this can be much faster than
count select from table where PERIOD=x
Please note that this computes a sum of booleans as a 32bit int, so if your table has more than a billion rows, you may want to add a cast:
exec sum "j"$PERIOD=x from table
The following will be the most efficient.
select count i from table where PERIOD=x
#jomahony solution will require all columns to be read from disk (if the table is on disk) before doing the count so can be inefficient
I am aggregation a table using file ID field. Each file has a name which matched exactly one (his) file id.
select file_key, min(fullfilepath)
from table
group by file_key
Because I know the structure of the table, I know that I need ANY fullfilepath. The min and the max are ok, but it requires a lot of time.
I came across this aggregation function which returns the first value. Unfortunately, this function takes a long time, because it scans the whole table. For example, this is very slow:
select first(file_id) from table;
What is the fastest way to do that? With or without aggregation function.
There is no way to make your first query with the GROUP BY clause faster, because it has to scan the whole table to find all groups.
Your second query can be made faster:
SELECT (
SELECT file_id FROM "table"
WHERE file_id IS NOT NULL
LIMIT 1
);
There is no way to optimize the query as you wrote it, because the aggregate function is a black box to PostgreSQL.
I doubt that this will help performance but it may be useful if anyone actually wants a first agregate.
-- coaslesce isn't a function so make an equivalent function.
create function coalesce_("anyelement","anyelement") returns "anyelement"
language sql as $$ select coalesce( $1,$2 ) $$;
create aggregate first("anyelement") (sfunc=coalesce_, stype="anyelement");
select
distinct on (file_key)
file_key, fullfilepath
from table
order by file_key
That will return one record for each file_key
I'm trying to get the whole result of a query into a variable, so I can loop through it and make inserts.
I don't know if it's possible.
I'm new to postgre and procedures, any help will be very welcome.
Something like:
declare result (I don't know what kind of data type I should use to get a query);
select into result label, number, desc from data
Thanks in advance!
I think you have to read PostgreSQL documentation about cursors.
But if you want just insert data from one table to another, you can do:
insert into data2 (label, number, desc)
select label, number, desc
from data
if you want to "save" data from query, you also can use temporary table, which you can create by usual create table or create table as:
create temporary table temp_data as
(
select label, number, desc
from data
)
see documentation
I have a table named categories, which contains ID(long), Name(varchar(50)), parentID(long), and shownByDefault(boolean) columns.
This table contains 554 records. All the shownByDefaultValues are 'false'.
When I execute 'select id, name from categories', pg returns me all the categories,
orderer by its id.
Then I update some of the rows of the table('update categories set shownByDefault where parentId = 1'), update OK.
Then, when I try to execute the first query, which returns all the categories, they are
returner with a very weird order.
I do not have problem to add 'order by', but since I am using JPA to get this values, anyone knows what the problem is or if there is a way to fix this?
That's not a problem. The order of rows returned by a SQL SELECT is undefined unless it has an ORDER BY. The order you get them is usually influenced by the order they are stored in the table and/or the indices that are used by the statement.
So depending on that order without using ORDER BY is a very, very bad idea.
If you need them in some order, simply specify that.
It is important that a table is a set of rows and not a sequence of rows.
From the docs:
If the ORDER BY clause is specified, the returned rows are sorted in the specified order. If ORDER BY is not given, the rows are returned in whatever order the system finds fastest to produce.
The rows are returned in whatever their physical order on disk is; you can reorder them physically using the CLUSTER SQL command, but due to the way Postgres works they'll become unordered as soon as you start modifying rows.
For what you're doing an ORDER BY is the right answer.