Which Is Faster/Best? Select * or Select Column1, Colum2, Column3, etc

Which is faster/best? SELECT * or SELECT column1, colum2, column3, etc

One reason that selecting specific columns is better is that it raises the probability that SQL Server can access the data from indexes rather than querying the table data.

Here's a post I wrote about it: The real reason select queries are bad index coverage

It's also less fragile to change, since any code that consumes the data will be getting the same data structure regardless of changes you make to the table schema in the future.

Which is faster/best? SELECT * or SELECT column1, colum2, column3, etc

One reason that selecting specific columns is better is that it raises the probability that SQL Server can access the data from indexes rather than querying the table data.

Here's a post I wrote about it: The real reason select queries are bad index coverage

It's also less fragile to change, since any code that consumes the data will be getting the same data structure regardless of changes you make to the table schema in the future.

Is there a performance difference between select * from tablename and select column1, column2 from tablename?

The answer is YES in general! Because for small databases you don't see a performance difference ... but with biggest databases could be relevant differences if you use the unqualified * selector as shorthand!

In general, it's better to instantiate each column from which you want to retrieve data!

I can suggest you to read the official document about how to optimize SELECT and other statements!

What does the expression : Select `(column1|column2|column3)?+.+` from Table in SQL means?

That is a way of selecting certain column names using regexps. That regex matches (and excludes) the columns column1, column2 or column3.

It is the Spark's equivalent of the Hive's Quoted Identifiers. See also Spark's documentation.

Be aware that, for enabling this behavior, it is first necessary to run the following command:

spark.sql("SET spark.sql.parser.quotedRegexColumnNames=true").show(false)

Is there a difference between Select * and Select [list each col]

Generally, it's better to be explicit, so Select col1, col2 from Table is better. The reason being that at some point, an extra column may be added to that table, and would cause unneeded data to be brought back from the query.

This isn't a hard and fast rule though.

Would be SELECT * faster than 40 columns listing?

Typing the columns out is faster for the database, typing * is faster for you to type. Pick whichever is more important :-)

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