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When and why are database joins expensive?
...nality when (if) they set up indexes.
It is important to understand that table scans (examination of every row in a table in the course of producing a join) are rare in practice. A query optimiser will choose a table scan only when one or more of the following holds.
There are fewer than 200 row...
How to create materialized views in SQL Server?
...ually I want to create a view and it should update automatically when base tables are changed. Can anyone explain with an query example..
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Difference Between Schema / Database in MySQL
...racle Database product, a schema represents only a part of a database: the tables and other objects owned by a single user.
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Google Chrome Printing Page Breaks
...g to spec break-after and break-before apply only to block-level elements, table row groups, table rows (but see prose): drafts.csswg.org/css-break-3/#break-between – that means no floats or any fancy positioning tricks.
– Mikko Rantalainen
Jun 2 '16 at 11:27...
PostgreSQL - how to quickly drop a user with existing privileges
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Doing: CREATE TABLE foo(bar SERIAL); ALTER TABLE foo OWNER TO postgres; CREATE USER testuser; GRANT ALL ON foo TO testuser; DROP USER testuser gave the error messages: ERROR: role "testuser" cannot be dropped because some objects depend...
Split comma-separated strings in a column into separate rows
...approach but modified according to Rich Scriven's comment,
Jaap's two data.table methods and two dplyr / tidyr approaches,
Ananda's splitstackshapesolution,
and two additional variants of Jaap's data.table methods.
Overall 8 different methods were benchmarked on 6 different sizes of data frames u...
Why are joins bad when considering scalability?
...ted comparison for a join is now much smaller and potentially pre-indexed.
Table partitions (helps with large data sets by spreading the load out to multiple disks, or limiting what might have been a table scan down to a partition scan)
OLAP (pre-computes results of certain kinds of queries/joins. I...
How do I find duplicate values in a table in Oracle?
...or a given column and the count of their occurrences in an Oracle database table?
13 Answers
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Splitting string into multiple rows in Oracle
...str(t.error, '[^,]+', 1, levels.column_value)) as error
from
temp t,
table(cast(multiset(select level from dual connect by level <= length (regexp_replace(t.error, '[^,]+')) + 1) as sys.OdciNumberList)) levels
order by name
EDIT:
Here is a simple (as in, "not in depth") explanation of t...
Oracle find a constraint
...stem generated constraint name. For instance, if we specify NOT NULL in a table declaration. Or indeed a primary or unique key. For example:
SQL> create table t23 (id number not null primary key)
2 /
Table created.
SQL> select constraint_name, constraint_type
2 from user_constraint...