# 19.15. 預先配置的參數

The following “parameters” are read-only, and are determined when PostgreSQL is compiled or when it is installed. 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.

`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](https://www.postgresql.org/docs/10/static/runtime-config-resource.html#GUC-SHARED-BUFFERS)) is influenced by `block_size`. See [Section 19.4](https://www.postgresql.org/docs/10/static/runtime-config-resource.html) for information.

`data_checksums` (`boolean`)

Reports whether data checksums are enabled for this cluster. See [data checksums](https://www.postgresql.org/docs/10/static/app-initdb.html#APP-INITDB-DATA-CHECKSUMS) 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`.

`lc_collate` (`string`)

Reports the locale in which sorting of textual data is done. See [Section 23.1](https://www.postgresql.org/docs/10/static/locale.html) for more information. This value is determined when a database is created.

`lc_ctype` (`string`)

Reports the locale that determines character classifications. See [Section 23.1](https://www.postgresql.org/docs/10/static/locale.html) 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](https://www.postgresql.org/docs/10/static/runtime-config-client.html#GUC-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.

`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 number of blocks (pages) in a WAL segment file. The total size of a WAL segment file in bytes is equal to `wal_segment_size` multiplied by `wal_block_size`; by default this is 16MB. See [Section 30.4](https://www.postgresql.org/docs/10/static/wal-configuration.html) for more information.