Running Queries

This topic provides a brief overview of query execution in Vertica. For detailed information about writing and executing queries, see the SQL Programmer's Guide.

Vertica can run any SQL query in snapshot isolation mode in order to obtain the fastest possible execution. To be precise, snapshot isolation mode is actually a form of

historical query. The syntax is:

Vertica can execute a query from a snapshot of the database taken at a specific date and time. The syntax is:

AT TIME 'timestamp' SELECT...

The command queries all data in the database up to and including the epoch representing the specified date and time without holding a lock or blocking write operations.

The command queries all data in the database up to but not including the current epoch without holding a lock or blocking write operations. This may cause the query to miss tuples loaded by other users up to (but no more than) a specific number of minutes before execution.

An epoch represents committed changes to the data stored in a database between two specific points in time. In other words, an epoch contains all COPY, INSERT, UPDATE, and DELETE operations that have been executed and committed since the end of the previous epoch.

Vertica can execute a query from a snapshot of the database taken at a specific date and time. The syntax is:

The command queries all data in the database up to and including the

epoch representing the specified date and time without holding a lock or blocking write operations.

For this reason, the DELETE command in Vertica does not actually delete data; it simply marks tuples as deleted. The UPDATE command actually does an INSERT and a DELETE.

The query optimizer chooses the projections to use for any given incoming query and forms an optimized query plan for the ones chosen. The query plan that the optimizer produces is decomposed into "mini-plans" which are run at various nodes in parallel, interspersed with data movement operations. A local executor at each node runs its assigned mini-plan.