In PostgreSQL, the query planner utilizes statistics and histograms to estimate the number of rows that will be returned by a given query, in order to optimize the plan for execution. The statistics and histograms are based on data collected by the PostgreSQL planner over time, with the goal of allowing it to make accurate estimates about how many rows a given query will return.
Statistics are collected on columns and indexes, including information such as the number of distinct values in a column, the minimum and maximum values, the most common values, and the distribution of values across the column. Histograms are a more detailed form of statistics, which divide the range of values in a column into buckets and record the number of rows that fall within each bucket.
For example, suppose we have a table of users, with columns for name, age, and gender. If we want to find the average age of all male users, the planner can use the statistics on the gender column to estimate how many rows will be returned for male users, and then use the histogram on the age column to estimate the average age of those users.
Influence on behavior: PostgreSQL provides a few ways to influence the behavior of the query planner with regards to statistics and histograms.
1. The ANALYZE command can be used to collect fresh statistics and histograms on a table or index.
2. The auto_explain module can be used to log the plans of slow or expensive queries, allowing us to examine how the planner is making use of statistics and histograms.
3. The statistics_target setting can be used to increase or decrease the level of detail in the statistics and histograms collected by the planner.
To conclude, statistics and histograms are a critical component of the PostgreSQL query planner, allowing it to make accurate estimates of the number of rows a query will return and optimizing the plan for execution. Developers can influence the behavior of the planner through various settings to ensure it uses the right data in the right way to optimize query execution.