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How does database indexing work? [closed]
... N/2 block accesses (on average), where N is the number of blocks that the table spans. If that field is a non-key field (i.e. doesn’t contain unique entries) then the entire tablespace must be searched at N block accesses.
Whereas with a sorted field, a Binary Search may be used, which has log2 ...
Add timestamps to an existing table
I need to add timestamps ( created_at & updated_at ) to an existing table. I tried the following code but it didn't work.
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Table is marked as crashed and should be repaired
...an use the command (there must be ample space for the mysql files)
REPAIR TABLE `<table name>`;
for repairing individual tables
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How to list of all the tables defined for the database when using active record?
How do I get a list of all the tables defined for the database when using active record?
5 Answers
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Make column not nullable in a Laravel migration
I'm writing a migration to make certain columns in a table nullable right now. For the down function, I of course want to make those columns not nullable again. I looked through the schema builder docs , but couldn't see a way to do this.
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How to add an auto-incrementing primary key to an existing table, in PostgreSQL?
I have a table with existing data. Is there a way to add a primary key without deleting and re-creating the table?
4 Answer...
jQuery delete all table rows except first
Using jQuery, how do I delete all rows in a table except the first? This is my first attempt at using index selectors. If I understand the examples correctly, the following should work:
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How do you delete a column by name in data.table?
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Any of the following will remove column foo from the data.table df3:
# Method 1 (and preferred as it takes 0.00s even on a 20GB data.table)
df3[,foo:=NULL]
df3[, c("foo","bar"):=NULL] # remove two columns
myVar = "foo"
df3[, (myVar):=NULL] # lookup myVar contents
# Method 2a ...
Can hash tables really be O(1)?
It seems to be common knowledge that hash tables can achieve O(1), but that has never made sense to me. Can someone please explain it? Here are two situations that come to mind:
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SQL WHERE ID IN (id1, id2, …, idn)
...DMS and so you would need us use @Ed Guiness's solution but here temporary tables do differ between RDBMS. (Effectively for complex problems you can't use just pure standard SQL)
– mmmmmm
Apr 27 '11 at 12:27
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