This is because the underlying data will change while executing the same queries. Usually, it’s more difficult to compare the timing of operations when doing INSERT, UPDATE, or DELETE using EXPLAIN ANALYZE. This is not only going to show you a query plan for deleting those rows, it is actually going to delete them. To actually execute the query we can use EXPLAIN ANALYZE as we’ll see later. The query is not executed, so it is safe to obtain the query plan. Consider the following statement: EXPLAIN DELETE FROM city If you instead use EXPLAIN ANALYZE before the statement, you’ll get both the estimation of what the planner expected, along with what actually happened when the query ran. This will show the query plan, in other words, the list of things expected to happen when that query is executed. If you have a slow query, the first thing to try is running it with EXPLAIN. Our goal will then be to observe what the optimizer thinks is the most expensive part of the query so that we could eliminate or enhance that part. Therefore, the optimizer has to make a lot of decisions, based on particular database statistics, before the query is executed. Still, if the client selects a large number of rows, this might not be valid anymore. Usually, this is the cheapest way to access a table. If the index has all the necessary data and there is no need to access the table, we have what’s called an Index Only Scan.īut that’s less often the case, so the index is used to filter out rows, and then accessing the table. In the second set of options, we access an index. Yet, this method works best for small tables. This is normally the most expensive because all rows must be fetched from the table and checked against a condition. The first method, fetching data directly from the table, is called Full Scan. Get the location of the records from the index, and go to the table to get the actual data.Stop here, because all the columns required by client are there in the index, or.Fetch the data directly from a table, or.For example, what is cheaper from a response time perspective? The optimizer has to make a few decisions before executing each query. The main tool is the EXPLAIN statement, which provides information about the query plan chosen by the optimizer. Once we have a candidate query for optimization, we need to analyze why it is slow, or why it impacts the system soo much.
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