53.5. pg_backend_memory_contexts
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## 53.5. pg_backend_memory_contexts #

The view pg_backend_memory_contexts displays all the memory contexts of the server process attached to the current session.

pg_backend_memory_contexts contains one row for each memory context.

Table 53.5. pg_backend_memory_contexts Columns

Column Type Description
`name` text Name of the memory context
`ident` text Identification information of the memory context. This field is truncated at 1024 bytes
`type` text Type of the memory context
`level` int4 The 1-based level of the context in the memory context hierarchy. The level of a context also shows the position of that context in the `path` column.
`path` int4[] Array of transient numerical identifiers to describe the memory context hierarchy. The first element is for `TopMemoryContext`, subsequent elements contain intermediate parents and the final element contains the identifier for the current context.
`total_bytes` int8 Total bytes allocated for this memory context
`total_nblocks` int8 Total number of blocks allocated for this memory context
`free_bytes` int8 Free space in bytes
`free_chunks` int8 Total number of free chunks
`used_bytes` int8 Used space in bytes

By default, the pg_backend_memory_contexts view can be read only by superusers or roles with the privileges of the `pg_read_all_stats` role.

Since memory contexts are created and destroyed during the running of a query, the identifiers stored in the `path` column can be unstable between multiple invocations of the view in the same query. The example below demonstrates an effective usage of this column and calculates the total number of bytes used by `CacheMemoryContext` and all of its children:

    WITH memory_contexts AS (
        SELECT * FROM pg_backend_memory_contexts
    )
    SELECT sum(c1.total_bytes)
    FROM memory_contexts c1, memory_contexts c2
    WHERE c2.name = 'CacheMemoryContext'
    AND c1.path[c2.level] = c2.path[c2.level];

The Common Table Expression is used to ensure the context IDs in the `path` column match between both evaluations of the view.

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