Check How Many Times a Given Query Plan was Chosen for a Prepared Statement in PostgreSQL

Posted on by Ian

When a prepared statement is created in PostgreSQL, the server has two possible strategies for executing it: use a generic plan or use a custom plan. Each time we run a prepared statement it will chose one of these two plans.

By default, it will use a custom plan for the first five runs. After that it will decide whether or not it’s more efficient to use a generic plan for subsequent executions, or stay with a custom plan.

This begs the question; how do we check to see which plan PostgreSQL chose?

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Possible Reason You’re Getting an Error When using a Qualified Table Name with MySQL’s HANDLER Statement

Posted on by Ian

If you’re getting MySQL error 1064 which reads something like “1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ‘READ FIRST’ at line 1” when using the HANDLER statement in MySQL, it could be that you’re qualifying the table with the database name when trying to read from or close a table.

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SQL Server OUTPUT Clause: Using Expressions to Analyze Changes to the Data

Posted on by Ian

When we use the OUTPUT clause in SQL Server, we have the option of providing expressions instead of just the column names. For example, we could use an expression that compares the old price with the new price and returns the difference. Such data could be handy, depending on what your goals are.

Let’s dive straight into an example that uses an expression in the OUTPUT clause.

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Fix “Ambiguous column name” in SQL Server (Error 209)

Posted on by Ian

If you’re getting an error that reads “Ambiguous column name“, followed by a column name, in SQL Server, it could be that you’re performing a join on two tables, but you’re not using the table names or aliases when referring to columns within those tables.

To fix this issue, be sure to qualify the columns with their table names or aliases.

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Fix “An aggregate may not appear in the OUTPUT clause” in SQL Server (Error 158)

Posted on by Ian

If you’re getting an error that reads “An aggregate may not appear in the OUTPUT clause” in SQL Server, it appears that you’re using an aggregate function in the OUTPUT clause.

Aggregate functions are not allowed in the OUTPUT clause.

To fix this issue, remove any aggregate functions from the OUTPUT clause and try applying them to a table or table variable.

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Using the INCLUDE Option on an Index in SQL Server

Posted on by Ian

The INCLUDE option in SQL Server allows us to include non-key columns in a nonclustered index. These columns are not part of the index key (which SQL Server uses to order and search the index), but they are stored with the index pages.

The INCLUDE option can significantly improve query performance when additional columns are needed by a query but are not part of the index key.

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Example of PostgreSQL Automatically Creating a Nested Composite Type

Posted on by Ian

Whenever we create a table in PostgreSQL, a composite type is automatically created behind the scenes. This composite type is based on the table that we created. Each column in the table becomes a field in the composite type.

If the table already uses a composite type, then the composite type that PostgreSQL creates will include that type in its definition, thereby creating a situation where we effectively have a nested composite type.

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Fix “Cannot insert the value NULL into column” in SQL Server (Error 515)

Posted on by Ian

If you’re getting an error that reads something like “Cannot insert the value NULL into column ‘CatId’, table ‘demo.dbo.Cats’; column does not allow nulls. INSERT fails” in SQL Server, it’s because you’re trying to insert a NULL value into non-nullable column (i.e. a column with a NOT NULL constraint).

We can’t insert NULL values into non-nullable columns.

To fix this issue, insert a non-NULL value.

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