In this video, learn how WHERE clauses affect query plans.
- [Instructor] Okay, let's continue to work … with the staff table. … And this time, we're going to restrict the rows … so that we return only those rows … that have a salary greater than $75,000. … So for that we're going to need to add a where clause … to that select statement we had earlier. … So I'll type select, start, from staff. … And I'm going to add a where clause, … where salary is greater than 75 thousand. … And we'll execute this. … And we'll notice there are 717 rows that returned. … So that's good to know. … We'll keep that in mind. … Now, let's see what steps were actually taken … to execute that statement. … So I'm going to insert explain, … so then I can, this will just estimate the cost. … It's not actually going to execute it. … And we'll get a query plan. … I'll just expand this out a little bit. … And we'll notice, this is very similar … to what we saw before. … We have a sequence scan, and we're returning about 700 rows. … And the width is about 75. … So you'll notice, the exact count on the rows is off by two. …
Author
Released
4/1/2019- How SQL executes queries
- Working with PostgreSQL tools for tuning
- Bitmap and hash indexes
- Using different types of indexes to improve performance
- Challenges with joining tables
- When to use partitioning to improve performance
- Collecting statistics about data in tables
Skill Level Advanced
Duration
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Introduction
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1. How SQL Executes a Query
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Scanning tables and indexes3m 12s
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Joining tables2m 24s
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Partitioning data2m 23s
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2. PostgreSQL Tools for Tuning
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Installing PostgreSQL2m 41s
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Overview of pgAdmin3m 21s
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Explain and analyze3m 37s
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Indexes3m 7s
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3. Types of Indexes
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Indexing3m 1s
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B-tree indexes2m 2s
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B-tree index example plan2m 18s
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Bitmap indexes1m 46s
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Bitmap index example plan2m 39s
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Hash indexes1m 19s
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Hash index example plan1m 32s
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PostgreSQL-specific indexes1m 17s
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4. Tuning Joins
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Nested loops3m 19s
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Nested loop example plan3m 55s
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Hash joins1m 25s
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Hash join example plan1m 57s
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Merge joins2m 17s
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Merge join example3m 16s
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Subqueries vs. joins1m 11s
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5. Partitioning Data
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Partition by range2m 6s
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Partition by list1m 37s
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Partition by list example4m 55s
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Partition by hash2m 12s
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Partition by hash example4m 51s
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6. Materialized Views
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Materialized views1m 41s
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7. Other Optimization Techniques
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Hints to the query optimizer2m 13s
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Parallel query execution2m 3s
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Miscellaneous tips1m 47s
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Conclusion
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Next steps41s
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Video: Example plan: Selecting with a WHERE clause