Tracking and maintaining expression statistics across database queries

One or more non-transitory computer-readable media storing instructions which, when executed by one or more processors, cause:

• maintaining, by a database server instance, an expression statistics store that includes metadata values for each expression of a plurality of expressions;
• wherein each expression of the plurality of expressions, when evaluated by the database server instance, causes the database server instance to perform a corresponding operation using values from at least one column of a table in a database managed by the database server instance;
• receiving, by the database server instance, a database query;
• parsing, by the database server instance, the database query to identify expressions within the database query;
• based on the parsing, identifying an occurrence of a particular expression within the database query;
• in response to identifying the occurrence of the particular expression within the database query, the database server instance updating the expression statistics store to reflect the occurrence of the particular expression;
• determining, from the metadata values maintained for the particular expression, a time-period-based cumulative cost for the particular expression;
• wherein the time-period-based cumulative cost is determined, based at least in part, on a fixed cost per evaluation of the particular expression and on an evaluation count that tracks how many times the particular expression was evaluated;
• based, at least in part, on the time-period-based cumulative cost for the particular expression, performing at least one of:
• storing an evaluation result for the particular expression in an in-memory cache, and modifying an execution plan of a query that includes the particular expression to access the evaluation results in the in-memory cache rather than evaluate the particular expression;
• rearranging an order in which expressions in a query that includes the particular expression are evaluated;
• changing a level of compression applied to a column or database object referenced in the particular expression;
• determining where to store a column or database object referenced in the particular expression;
• or
• determining a format for storing a column or database object referenced in the particular expression.