Pragmas

The PRAGMA statement is a SQL extension adopted by SereneDB from SQLite. PRAGMA statements can be issued in a similar manner to regular SQL statements. PRAGMA commands may alter the internal state of the database engine, and can influence the subsequent execution or behavior of the engine.

PRAGMA statements that assign a value to an option can also be issued using the SET statement and the value of an option can be retrieved using SELECT current_setting(option_name).

For SereneDB's built in configuration options, see the Configuration Reference.

This page contains the supported PRAGMA settings.

Metadata

Schema Information

List all databases:

Query

PRAGMA database_list;

List all tables:

Query

PRAGMA show_tables;

List all tables, with extra information, similarly to DESCRIBE:

Query

PRAGMA show_tables_expanded;

To list all functions:

Query

PRAGMA functions;

For queries targeting non-existing schemas, SereneDB generates “did you mean...” style error messages. When there are thousands of attached databases, these errors can take a long time to generate. To limit the number of schemas SereneDB looks through, use the catalog_error_max_schemas option:

Query

SET catalog_error_max_schemas = 10;

Table Information

Get info for a specific table:

Query

PRAGMA table_info('table_name');
CALL pragma_table_info('table_name');

Result

 cid | name | type    | notnull | dflt_value | pk-----+------+---------+---------+------------+----   0 | id   | INTEGER | f       | NULL       | f   1 | name | VARCHAR | f       | NULL       | f
 cid | name | type    | notnull | dflt_value | pk-----+------+---------+---------+------------+----   0 | id   | INTEGER | f       | NULL       | f   1 | name | VARCHAR | f       | NULL       | f

table_info returns information about the columns of the table with name table_name. The exact format of the table returned is given below:

Query

cid INTEGER,        -- cid of the columnname VARCHAR,       -- name of the columntype VARCHAR,       -- type of the columnnotnull BOOLEAN,    -- if the column is marked as NOT NULLdflt_value VARCHAR, -- default value of the column, or NULL if not specifiedpk BOOLEAN          -- part of the primary key or not

Database Size

Get the file and memory size of each database:

Query

PRAGMA database_size;
CALL pragma_database_size();

database_size returns information about the file and memory size of each database. The column types of the returned results are given below:

Query

database_name VARCHAR, -- database namedatabase_size VARCHAR, -- total block count times the block sizeblock_size BIGINT,     -- database block sizetotal_blocks BIGINT,   -- total blocks in the databaseused_blocks BIGINT,    -- used blocks in the databasefree_blocks BIGINT,    -- free blocks in the databasewal_size VARCHAR,      -- write ahead log sizememory_usage VARCHAR,  -- memory used by the database buffer managermemory_limit VARCHAR   -- maximum memory allowed for the database

Storage Information

To get storage information:

Query

PRAGMA storage_info('table_name');
CALL pragma_storage_info('table_name');

Result

row_group_id	column_name	column_id	column_path	segment_id	segment_type	start	count	compression	stats	has_updates	persistent	block_id	block_offset	additional_block_ids
row_group_id	column_name	column_id	column_path	segment_id	segment_type	start	count	compression	stats	has_updates	persistent	block_id	block_offset	additional_block_ids

This call returns the following information for the given table:

Name	Type	Description
row_group_id	BIGINT
column_name	VARCHAR
column_id	BIGINT
column_path	VARCHAR
segment_id	BIGINT
segment_type	VARCHAR
start	BIGINT	The start row id of this chunk
count	BIGINT	The amount of entries in this storage chunk
compression	VARCHAR	Compression type used for this column
stats	VARCHAR
has_updates	BOOLEAN
persistent	BOOLEAN	false if temporary table
block_id	BIGINT	Empty unless persistent
block_offset	BIGINT	Empty unless persistent

Show Databases

The following statement is equivalent to the SHOW DATABASES statement:

Query

PRAGMA show_databases;

Resource Management

Memory Limit

Set the memory limit for the buffer manager:

Query

SET memory_limit = '1GB';

Attention

The specified memory limit is only applied to the buffer manager. For most queries, the buffer manager handles the majority of the data processed. However, certain in-memory data structures such as vectors and query results are allocated outside of the buffer manager. Additionally, aggregate functions with complex state (e.g., list, mode, quantile, string_agg, and approx functions) use memory outside of the buffer manager. Therefore, the actual memory consumption can be higher than the specified memory limit.

Threads

Set the amount of threads for parallel query execution:

Query

SET threads = 4;

Collations

List all available collations:

Query

PRAGMA collations;

Set the default collation to one of the available ones:

Query

SET default_collation = 'nocase';

Default Ordering for NULLs

Set the default ordering for NULLs to be either NULLS_FIRST, NULLS_LAST, NULLS_FIRST_ON_ASC_LAST_ON_DESC or NULLS_LAST_ON_ASC_FIRST_ON_DESC:

Query

SET default_null_order = 'NULLS_FIRST';
SET default_null_order = 'NULLS_LAST_ON_ASC_FIRST_ON_DESC';

Set the default result set ordering direction to ASCENDING or DESCENDING:

Query

SET default_order = 'ASCENDING';
SET default_order = 'DESCENDING';

Ordering by Non-Integer Literals

By default, ordering by non-integer literals is not allowed:

Query

SELECT 42 ORDER BY 'hello world';

Result

db error: ERROR: ORDER BY non-integer literal has no effect.* SET order_by_non_integer_literal=true to allow this behavior.

To allow this behavior, use the order_by_non_integer_literal option:

Query

SET order_by_non_integer_literal = true;

Information on SereneDB

Version

Show SereneDB version:

Query

PRAGMA version;

Result

 library_version | source_id  | codename-----------------+------------+--------------------- v1.6.0-dev7012  | 5212023fb8 | Development Version

Platform

platform returns an identifier for the platform the current SereneDB executable has been compiled for, e.g., osx_arm64. The format of this identifier matches the platform name:

Query

PRAGMA platform;
CALL pragma_platform();

Result

 platform---------- (varies)
 platform---------- (varies)

User Agent

The following statement returns the user agent information, e.g., duckdb/v0.0.1(linux_arm64) cpp:

Query

PRAGMA user_agent;

Result

 user_agent---------------------------------------- duckdb/v1.6.0-dev7012(linux_amd64) cpp

Metadata Information

The following statement returns information on the metadata store (block_id, total_blocks, free_blocks, and free_list):

Query

PRAGMA metadata_info;

Result

 block_id | total_blocks | free_blocks | free_list----------+--------------+-------------+-----------        0 |           64 |           2 |         1

Progress Bar

Show progress bar when running queries:

Query

PRAGMA enable_progress_bar;

Result

Success

Or:

Query

PRAGMA enable_print_progress_bar;

Result

Success

Don't show a progress bar for running queries:

Query

PRAGMA disable_progress_bar;

Result

Success

Or:

Query

PRAGMA disable_print_progress_bar;

Result

Success

EXPLAIN Output

The output of EXPLAIN can be configured to show only the physical plan.

The default configuration of EXPLAIN:

Query

SET explain_output = 'physical_only';

To only show the optimized query plan:

Query

SET explain_output = 'optimized_only';

To show all query plans:

Query

SET explain_output = 'all';

Profiling

Enable Profiling

The following query enables profiling with the default format, query_tree. Independent of the format, enable_profiling is mandatory to enable profiling.

Query

PRAGMA enable_profiling;
PRAGMA enable_profile;

Result

Success
Success

Profiling Coverage

By default, the profiling coverage is set to SELECT. SELECT runs the profiler for each operator in the physical plan of a SELECT statement.

Query

SET profiling_coverage = 'SELECT';

By default, the profiler does not emit profiling information for other statement types (INSERT INTO, ATTACH, etc.). To run the profiler for all statement types, change this setting to ALL.

Query

SET profiling_coverage = 'ALL';

Profiling Format

The format of enable_profiling can be specified as query_tree, json, query_tree_optimizer, or no_output. Each format prints its output to the configured output, except no_output.

The default format is query_tree. It prints the physical query plan and the metrics of each operator in the tree.

Query

SET enable_profiling = 'query_tree';

Alternatively, json returns the physical query plan as JSON:

Query

SET enable_profiling = 'json';

To return the physical query plan, including optimizer and planner metrics:

Query

SET enable_profiling = 'query_tree_optimizer';

Database drivers and other applications can also access profiling information through API calls, in which case users can disable any other output. Even though the parameter reads no_output, it is essential to note that this only affects printing to the configurable output. When accessing profiling information through API calls, it is still crucial to enable profiling:

Query

SET enable_profiling = 'no_output';

Profiling Output

By default, SereneDB prints profiling information to the standard output. However, if you prefer to write the profiling information to a file, you can use PRAGMA profiling_output to specify a filepath.

Attention

The file contents will be overwritten for every newly issued query. Hence, the file will only contain the profiling information of the last run query:

Query

SET profiling_output = '/path/to/file.json';
SET profile_output = '/path/to/file.json';

Profiling Mode

By default, a limited amount of profiling information is provided (standard).

Query

SET profiling_mode = 'standard';

For more details, use the detailed profiling mode by setting profiling_mode to detailed. The output of this mode includes profiling of the planner and optimizer stages.

Query

SET profiling_mode = 'detailed';

To access all available metrics, set the profiling_mode to all.

Query

SET profiling_mode = 'all';

Disable Profiling

To disable profiling:

Query

PRAGMA disable_profiling;
PRAGMA disable_profile;

Result

Success
Success

Query Optimization

Optimizer

To disable the query optimizer:

Query

PRAGMA disable_optimizer;

Result

Success

To enable the query optimizer:

Query

PRAGMA enable_optimizer;

Result

Success

Selectively Disabling Optimizers

The disabled_optimizers option allows selectively disabling optimization steps. For example, to disable filter_pushdown and statistics_propagation, run:

Query

SET disabled_optimizers = 'filter_pushdown,statistics_propagation';

The available optimizations can be queried using the duckdb_optimizers() table function.

To re-enable the optimizers, run:

Query

SET disabled_optimizers = '';

Attention

The disabled_optimizers option should only be used for debugging performance issues and should be avoided in production.

Logging

Set a path for query logging:

Query

SET log_query_path = '${__TEST_DIR__}/serened_log/';

Disable query logging:

Query

SET log_query_path = '';

Object Cache

Enable caching of objects for e.g., Parquet metadata:

Query

PRAGMA enable_object_cache;

Result

Success

Disable caching of objects:

Query

PRAGMA disable_object_cache;

Result

Success

Checkpointing

Compression

During checkpointing, the existing column data + any new changes get compressed. There exist a couple of pragmas to influence which compression functions are considered.

Force Compression

Prefer using this compression method over any other method if possible:

Query

PRAGMA force_compression = 'bitpacking';

Disabled Compression Methods

Avoid using any of the listed compression methods from the comma separated list:

Query

PRAGMA disabled_compression_methods = 'fsst,rle';

Force Checkpoint

When CHECKPOINT is called when no changes are made, force a checkpoint regardless:

Query

PRAGMA force_checkpoint;

Result

Success

Checkpoint on Shutdown

Run a CHECKPOINT on successful shutdown and delete the WAL, to leave only a single database file behind:

Query

PRAGMA enable_checkpoint_on_shutdown;

Result

Success

Don't run a CHECKPOINT on shutdown:

Query

PRAGMA disable_checkpoint_on_shutdown;

Result

Success

Temp Directory for Spilling Data to Disk

By default, SereneDB uses a temporary directory named ⟨database_file_name⟩.tmp to spill to disk, located in the same directory as the database file. To change this, use:

Query

SET temp_directory = '/path/to/temp_dir.tmp/';

Returning Errors as JSON

The errors_as_json setting makes the serened shell report errors as raw JSON instead of a formatted message, which is easier to process programmatically. For certain errors it includes extra, decomposed fields:

Query

SET errors_as_json = true;

With the setting enabled, a failing query in the shell prints a JSON object such as:

{"exception_type":"Catalog","exception_message":"Table with name nonexistent_tbl does not exist!","type":"Table","name":"nonexistent_tbl","error_subtype":"MISSING_ENTRY"}

Over the PostgreSQL wire protocol, errors are always returned as standard PostgreSQL error messages, regardless of this setting.

The JSON error format applies to SereneDB's native interface. When connected over the PostgreSQL wire protocol (for example via psql or any PostgreSQL driver), errors are always delivered through the standard PostgreSQL error fields, so errors_as_json has no visible effect on the client.

IEEE Floating-Point Operation Semantics

SereneDB follows IEEE floating-point operation semantics. If you would like to turn this off, run:

Query

SET ieee_floating_point_ops = false;

In this case, floating point division by zero (e.g., 1.0 / 0.0, 0.0 / 0.0 and -1.0 / 0.0) will all return NULL.

Query Verification (for Development)

The following PRAGMAs are mostly used for development and internal testing.

Enable query verification:

Query

PRAGMA enable_verification;

Result

Success

Disable query verification:

Query

PRAGMA disable_verification;

Result

Success

Enable force parallel query processing:

Query

PRAGMA verify_parallelism;

Result

Success

Disable force parallel query processing:

Query

PRAGMA disable_verify_parallelism;

Result

Success

Block Sizes

When persisting a database to disk, SereneDB writes to a dedicated file containing a list of blocks holding the data. In the case of a file that only holds very little data, e.g., a small table, the default block size of 256 kB might not be ideal. Therefore, SereneDB's storage format supports different block sizes.

There are a few constraints on possible block size values.

• Must be a power of two.
• Must be greater or equal to 16384 (16 kB).
• Must be lesser or equal to 262144 (256 kB).

You can set the default block size for all new SereneDB files created by an instance like so:

Query

SET default_block_size = '16384';

It is also possible to set the block size on a per-file basis, see ATTACH for details.