20.15. Preset Options The following “parameters” are read-only. As such, they have been excluded from the sample postgresql.conf file. These options report various aspects of PostgreSQL behavior that might be of interest to certain applications, particularly administrative front-ends. Most of them are determined when PostgreSQL is compiled or when it is installed.   
block_size (integer)   Reports the size of a disk block. It is determined by the value of BLCKSZ when building the server. The default value is 8192 bytes. The meaning of some configuration variables (such as shared_buffers) is influenced by block_size. See Section 20.4 for information.  
data_checksums (boolean)   Reports whether data checksums are enabled for this cluster. See data checksums for more information.  
data_directory_mode (integer)   On Unix systems this parameter reports the permissions the data directory (defined by data_directory) had at server startup. (On Microsoft Windows this parameter will always display 0700.) See group access for more information.  
debug_assertions (boolean)   Reports whether PostgreSQL has been built with assertions enabled. That is the case if the macro USE_ASSERT_CHECKING is defined when PostgreSQL is built (accomplished e.g., by the configure option --enable-cassert). By default PostgreSQL is built without assertions.  
integer_datetimes (boolean)   Reports whether PostgreSQL was built with support for 64-bit-integer dates and times. As of PostgreSQL 10, this is always on.  
in_hot_standby (boolean)   Reports whether the server is currently in hot standby mode. When this is on, all transactions are forced to be read-only. Within a session, this can change only if the server is promoted to be primary. See Section 27.4 for more information.  
lc_collate (string)   Reports the locale in which sorting of textual data is done. See Section 24.1 for more information. This value is determined when a database is created.  
lc_ctype (string)   Reports the locale that determines character classifications. See Section 24.1 for more information. This value is determined when a database is created. Ordinarily this will be the same as lc_collate, but for special applications it might be set differently.  
max_function_args (integer)   Reports the maximum number of function arguments. It is determined by the value of FUNC_MAX_ARGS when building the server. The default value is 100 arguments.  
max_identifier_length (integer)   Reports the maximum identifier length. It is determined as one less than the value of NAMEDATALEN when building the server. The default value of NAMEDATALEN is 64; therefore the default max_identifier_length is 63 bytes, which can be less than 63 characters when using multibyte encodings.  
max_index_keys (integer)   Reports the maximum number of index keys. It is determined by the value of INDEX_MAX_KEYS when building the server. The default value is 32 keys.  
segment_size (integer)   Reports the number of blocks (pages) that can be stored within a file segment. It is determined by the value of RELSEG_SIZE when building the server. The maximum size of a segment file in bytes is equal to segment_size multiplied by block_size; by default this is 1GB.  
server_encoding (string)    Reports the database encoding (character set). It is determined when the database is created. Ordinarily, clients need only be concerned with the value of client_encoding.  
server_version (string)   Reports the version number of the server. It is determined by the value of PG_VERSION when building the server.  
server_version_num (integer)   Reports the version number of the server as an integer. It is determined by the value of PG_VERSION_NUM when building the server.  
shared_memory_size (integer)   Reports the size of the main shared memory area, rounded up to the nearest megabyte.  
shared_memory_size_in_huge_pages (integer)   Reports the number of huge pages that are needed for the main shared memory area based on the specified huge_page_size. If huge pages are not supported, this will be -1. This setting is supported only on Linux. It is always set to -1 on other platforms. For more details about using huge pages on Linux, see Section 19.4.5.  
ssl_library (string)   Reports the name of the SSL library that this PostgreSQL server was built with (even if SSL is not currently configured or in use on this instance), for example OpenSSL, or an empty string if none.  
wal_block_size (integer)   Reports the size of a WAL disk block. It is determined by the value of XLOG_BLCKSZ when building the server. The default value is 8192 bytes.  
wal_segment_size (integer)   Reports the size of write ahead log segments. The default value is 16MB. See Section 30.5 for more information.   

    
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