Not necessarily. MAX(x) can use a B-tree index when PostgreSQL estimates that an ordered index path suits the full query, but it does not guarantee one. Likewise, FILTER (WHERE ...) limits the rows fed to its aggregate; it does not, by itself, require a table scan. Check the plan for the exact query with EXPLAIN.
What MAX and aggregate FILTER do
MAX(x) returns the greatest non-null input value. PostgreSQL supports it for numeric, string, date/time, enum, and other sortable types. An aggregate filter changes the inputs to the particular aggregate that carries it: only rows for which the condition is true are included.
As the PostgreSQL 18 documentation puts it: “If FILTER is specified, then only the input rows for which the filter_clause evaluates to true are fed to the aggregate function; other rows are discarded.” PostgreSQL 18: Aggregate Expressions
Aggregate FILTER is not the same as query-level WHERE
A query-level WHERE clause removes rows from the query’s input, affecting every aggregate at that query level. An aggregate-level FILTER narrows the input to just the aggregate that has the clause. Other aggregates in the same select list can still receive the unfiltered rows. PostgreSQL’s aggregate documentation demonstrates this distinction with filtered and unfiltered counts. PostgreSQL 16 tutorial: Aggregate Functions
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For example, these two queries can return the same scalar when there is only one aggregate and no other relevant query structure:
-- The query-level WHERE restricts rows available to the aggregate.
SELECT max(x)
FROM measurements
WHERE active;
-- FILTER restricts input to this aggregate only.
SELECT max(x) FILTER (WHERE active)
FROM measurements;They are not interchangeable in every query. With multiple aggregates, grouping, or other output, a query-level restriction may change results beyond the conditional maximum.
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Why an index may help—but is not guaranteed
PostgreSQL B-tree indexes can support ordered retrieval, so an index on x may offer a useful path for finding a maximum. That is an option for the planner, not a promise that it will use the index. The query’s other predicates, index definition, table size, statistics, and estimated costs all matter. PostgreSQL also notes that reading sorted values from an index is not always faster than scanning and sorting. PostgreSQL 18: Index Types
The same caution applies to MAX(x) FILTER (WHERE active). The filter specifies which rows count toward the aggregate; the SQL expression alone does not tell you whether PostgreSQL will use an index or visit the table with a sequential scan. The plan depends on the exact query and database conditions.
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How to check the plan for your query
- Run
EXPLAINon the exact query you care about:EXPLAIN SELECT max(x) FILTER (WHERE active) FROM measurements; - Read the plan nodes to see whether PostgreSQL chose a sequential scan, an index scan, or another path, and which operations appear above the scan.
- If you need measured execution information, use
EXPLAIN ANALYZE. It executes the query, so take care with statements that have side effects. PostgreSQL 18: Using EXPLAIN
Compare alternative query forms using the same data, schema, statistics, and PostgreSQL version. A plan from one database does not establish what another database will do.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to interpret a sequential scan
A sequential scan with a filter visits table rows and tests the condition; the plan reports that scan and the operations built on top of it. This can be the planner’s selected path even when an index exists. The presence of MAX or aggregate FILTER is not enough to diagnose why: inspect the actual plan rather than inferring the access method from the SQL syntax. PostgreSQL 18: Using EXPLAIN
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