Viewing Table and Database Information

Querying Table Information

• Querying information about all tables in a database using the pg_tables system catalog

  1	SELECT * FROM pg_tables;

• Querying the table structure using \d+ command of the gsql tool.
  Example: Create a table customer_t1 and insert data into the table.

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  CREATE TABLE customer_t1 ( c_customer_sk integer, c_customer_id char(5), c_first_name char(6), c_last_name char(8) ) with (orientation = column,compression=middle) distribute by hash (c_last_name);

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  INSERT INTO customer_t1 (c_customer_sk, c_customer_id, c_first_name) VALUES (6885, 'map', 'Peter'), (4321, 'river', 'Lily'), (9527, 'world', 'James');

  Query the table structure. If no schema is specified when you create a table, the schema of the table defaults to public.

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  \d+ customer_t1; Table "public.customer_t1" Column | Type | Modifiers | Storage | Stats target | Description ---------------+--------------+-----------+----------+--------------+------------- c_customer_sk | integer | | plain | | c_customer_id | character(5) | | extended | | c_first_name | character(6) | | extended | | c_last_name | character(8) | | extended | | Has OIDs: no Distribute By: HASH(c_last_name) Location Nodes: ALL DATANODES Options: orientation=column, compression=middle, colversion=2.0, enable_delta=false

  

  The options may vary in different versions but the difference does not affect services. The options here are for reference only. The actual options are subject to the version.

• Use pg_get_tabledef to query the table definition.

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  SELECT * FROM PG_GET_TABLEDEF('customer_t1'); pg_get_tabledef ----------------------------------------------------------------------------------- SET search_path = tpchobs; + CREATE TABLE customer_t1 ( + c_customer_sk integer, + c_customer_id character(5), + c_first_name character(6), + c_last_name character(8) + ) + WITH (orientation=column, compression=middle, colversion=2.0, enable_delta=false)+ DISTRIBUTE BY HASH(c_last_name) + TO GROUP group_version1; (1 row)

• Querying all data in customer_t1

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  SELECT * FROM customer_t1; c_customer_sk | c_customer_id | c_first_name | c_last_name ---------------+---------------+--------------+------------- 6885 | map | Peter | 4321 | river | Lily | 9527 | world | James | (3 rows)

• Querying all data of a column in customer_t1 using SELECT

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  SELECT c_customer_sk FROM customer_t1; c_customer_sk --------------- 6885 4321 9527 (3 rows)

• Check whether a table has been analyzed. The time when the table was analyzed will be returned. If nothing is returned, it indicates that the table has not been analyzed.

  1	SELECT pg_stat_get_last_analyze_time(oid),relname FROM pg_class where relkind='r';

  Query the time when the public table was analyzed.

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  SELECT pg_stat_get_last_analyze_time(c.oid),c.relname FROM pg_class c LEFT JOIN pg_namespace n ON c.relnamespace = n.oid WHERE c.relkind='r' AND n.nspname='public'; pg_stat_get_last_analyze_time | relname -------------------------------+---------------------- 2022-05-17 07:48:26.923782+00 | warehouse_t19 2022-05-17 07:48:26.964512+00 | emp 2022-05-17 07:48:27.016709+00 | test_trigger_src_tbl 2022-05-17 07:48:27.045385+00 | customer 2022-05-17 07:48:27.062486+00 | warehouse_t1 2022-05-17 07:48:27.114884+00 | customer_t1 2022-05-17 07:48:27.172256+00 | product_info_input 2022-05-17 07:48:27.197014+00 | tt1 2022-05-17 07:48:27.212906+00 | timezone_test (9 rows)

• Quickly query the column information of a table. If a view in information_schema has a large number of objects in the database, it takes a long time to return the result. You can run the following SQL statement to quickly query the column information of one or more tables:

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  SELECT /*+ nestloop(a c)*/ c.column_name, c.data_type, c.ordinal_position, pgd.description, pp.partkey, c.is_nullable, c.column_default, c.character_maximum_length, c.numeric_precision, c.numeric_scale, c.datetime_precision, c.interval_type, c.udt_name from information_schema.columns as c left join pg_namespace sp on sp.nspname = c.table_schema left join pg_class cla on cla.relname = c.table_name and cla.relnamespace = sp.oid left join pg_catalog.pg_partition pp on (pp.parentid = cla.oid and pp.parttype = 'r') left join pg_catalog.pg_description pgd on (pgd.objoid=cla.oid and pgd.objsubid = c.ordinal_position)where c.table_name in ('tablename') and c.table_schema = 'public';

  For example, to quickly query the column information of the customer_t1 table, run the following command:

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  SELECT /*+ nestloop(a c)*/ c.column_name, c.data_type, c.ordinal_position, pgd.description, pp.partkey, c.is_nullable, c.column_default, c.character_maximum_length, c.numeric_precision, c.numeric_scale, c.datetime_precision, c.interval_type, c.udt_name from information_schema.columns as c left join pg_namespace sp on sp.nspname = c.table_schema left join pg_class cla on cla.relname = c.table_name and cla.relnamespace = sp.oid left join pg_catalog.pg_partition pp on (pp.parentid = cla.oid and pp.parttype = 'r') left join pg_catalog.pg_description pgd on (pgd.objoid=cla.oid and pgd.objsubid = c.ordinal_position) where c.table_name in ('customer_t1') and c.table_schema = 'public'; column_name | data_type | ordinal_position | description | partkey | is_nullable | column_default | character_maximum_length | numeric_precision | numeric_scale | datetime_precision | interval_type | udt_name ---------------+-----------+------------------+-------------+---------+-------------+----------------+--------------------------+-------------------+------------- --+--------------------+---------------+---------- c_last_name | character | 4 | | | YES | | 8 | | | | | bpchar c_first_name | character | 3 | | | YES | | 6 | | | | | bpchar c_customer_id | character | 2 | | | YES | | 5 | | | | | bpchar c_customer_sk | integer | 1 | | | YES | | | 32 | 0 | | | int4 (4 rows)

• Obtain the table definition by querying audit logs.

  Use the pgxc_query_audit function to query audit logs of all CNs. The syntax is as follows:

  1	pgxc_query_audit(timestamptz startime,timestamptz endtime)

  Query the audit records of multiple objects.

  1 2	SET audit_object_name_format TO 'all'; SELECT object_name,result,operation_type,command_text FROM pgxc_query_audit('2022-08-26 8:00:00','2022-08-26 22:55:00') where command_text like '%student%';

Querying the Table Size

• Querying the total size of a table (indexes and data included)

  1	SELECT pg_size_pretty(pg_total_relation_size('<schemaname>.<tablename>'));

  Example:

  First, create an index on customer_t1.

  1	CREATE INDEX index1 ON customer_t1 USING btree(c_customer_sk);

  Then, query the size of table customer_t1 of public.

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  SELECT pg_size_pretty(pg_total_relation_size('public.customer_t1')); pg_size_pretty ---------------- 264 kB (1 row)

• Querying the size of a table (indexes excluded)

  1	SELECT pg_size_pretty(pg_relation_size('<schemaname>.<tablename>'));

  Example: Query the size of table customer_t1 of public.

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  SELECT pg_size_pretty(pg_relation_size('public.customer_t1')); pg_size_pretty ---------------- 208 kB (1 row)

• Query all the tables, ranked by their occupied space.

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  SELECT table_schema || '.' || table_name AS table_full_name, pg_size_pretty(pg_total_relation_size('"' || table_schema || '"."' || table_name || '"')) AS size FROM information_schema.tables ORDER BY pg_total_relation_size('"' || table_schema || '"."' || table_name || '"') DESC limit xx;

  Example 1: Query the 15 tables that occupy the most space.

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  SELECT table_schema || '.' || table_name AS table_full_name, pg_size_pretty(pg_total_relation_size('"' || table_schema || '"."' || table_name || '"')) AS size FROM information_schema.tables ORDER BY pg_total_relation_size('"' || table_schema || '"."' || table_name || '"') DESC limit 15; table_full_name | size ---------------------------+--------- pg_catalog.pg_attribute | 2048 KB pg_catalog.pg_rewrite | 1888 KB pg_catalog.pg_depend | 1464 KB pg_catalog.pg_proc | 1464 KB pg_catalog.pg_class | 512 KB pg_catalog.pg_description | 504 KB pg_catalog.pg_collation | 360 KB pg_catalog.pg_statistic | 352 KB pg_catalog.pg_type | 344 KB pg_catalog.pg_operator | 224 KB pg_catalog.pg_amop | 208 KB public.tt1 | 160 KB pg_catalog.pg_amproc | 120 KB pg_catalog.pg_index | 120 KB pg_catalog.pg_constraint | 112 KB (15 rows)

  Example 2: Query the top 20 tables with the largest space usage in the public schema.

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  SELECT table_schema || '.' || table_name AS table_full_name, pg_size_pretty(pg_total_relation_size('"' || table_schema || '"."' || table_name || '"')) AS size FROM information_schema.tables where table_schema='public' ORDER BY pg_total_relation_size('"' || table_schema || '"."' || table_name || '"') DESC limit 20; table_full_name | size -----------------------------+--------- public.tt1 | 160 KB public.product_info_input | 112 KB public.customer_t1 | 96 KB public.warehouse_t19 | 48 KB public.emp | 32 KB public.customer | 0 bytes public.test_trigger_src_tbl | 0 bytes public.warehouse_t1 | 0 bytes (8 rows)

Quickly Querying the Space Occupied by All Tables in the Database

In a large cluster with a large amount of data (more than 1000 tables), you are advised to use the pgxc_wlm_table_distribution_skewness view to query all tables in the database. This view can be used to query the tablespace usage and data skew distribution in the database. The unit of total_size and avg_size is byte.

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SELECT *, pg_size_pretty(total_size) as tableSize FROM pgxc_wlm_table_distribution_skewness ORDER BY total_size desc;
    schema_name     |                    table_name                     | total_size | avg_size  | max_percent | min_percent | skew_percent | tablesize 
--------------------+---------------------------------------------------+------------+-----------+-------------+-------------+--------------+-----------
 public             | history_tbs_test_row_1                            |  804347904 | 134057984 |       18.02 |       15.63 |         7.53 | 767 MB
 public             | history_tbs_test_row_3                            |  402096128 |  67016021 |       18.30 |       15.60 |         8.90 | 383 MB
 public             | history_tbs_test_row_2                            |  401743872 |  66957312 |       18.01 |       15.01 |         7.47 | 383 MB
 public             | i_history_tbs_test_1                              |  325263360 |  54210560 |       17.90 |       15.50 |         6.90 | 310 MB

The query result shows that the history_tbs_test_row_1 table occupies the largest space and data skew occurs.

1. The pgxc_wlm_table_distribution_skewness view can be queried only when the GUC parameters use_workload_manager and enable_perm_space are enabled. In earlier versions, you are advised to use the table_distribution() function to query the entire database. If only the size of a table is queried, the table_distribution(schemaname text, tablename text) function is recommended.
2. In 8.2.1 and later cluster versions, GaussDB (DWS) supports the pgxc_wlm_table_distribution_skewness view, which can be directly queried.
3. In the 8.1.3 cluster version, you can use the following definition to create a view and then query the view:

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CREATE OR REPLACE VIEW pgxc_wlm_table_distribution_skewness AS WITH skew AS ( SELECT schemaname, tablename, pg_catalog.sum(dnsize) AS totalsize, pg_catalog.avg(dnsize) AS avgsize, pg_catalog.max(dnsize) AS maxsize, pg_catalog.min(dnsize) AS minsize, (maxsize - avgsize) * 100 AS skewsize FROM pg_catalog.gs_table_distribution() GROUP BY schemaname, tablename ) SELECT schemaname AS schema_name, tablename AS table_name, totalsize AS total_size, avgsize::numeric(1000) AS avg_size, ( CASE WHEN totalsize = 0 THEN 0.00 ELSE (maxsize * 100 / totalsize)::numeric(5, 2) END ) AS max_percent, ( CASE WHEN totalsize = 0 THEN 0.00 ELSE (minsize * 100 / totalsize)::numeric(5, 2) END ) AS min_percent, ( CASE WHEN totalsize = 0 THEN 0.00 ELSE (skewsize / maxsize)::numeric(5, 2) END ) AS skew_percent FROM skew;

Querying Database Information

• Querying the database list using the \l meta-command of the gsql tool.

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  \l List of databases Name | Owner | Encoding | Collate | Ctype | Access privileges -----------+-------+-----------+---------+-------+------------------- gaussdb | Ruby | SQL_ASCII | C | C | template0 | Ruby | SQL_ASCII | C | C | =c/Ruby + | | | | | Ruby=CTc/Ruby template1 | Ruby | SQL_ASCII | C | C | =c/Ruby + | | | | | Ruby=CTc/Ruby (3 rows)

  

  • If the parameters LC_COLLATE and LC_CTYPE are not specified during database installation, the default values of them are C.
  • If LC_COLLATE and LC_CTYPE are not specified during database creation, the sorting order and character classification of the template database are used by default.

    For details, see CREATE DATABASE.

• Querying the database list using the pg_database system catalog

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  SELECT datname FROM pg_database; datname ----------- template1 template0 gaussdb (3 rows)

Querying the Database Size

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select datname,pg_size_pretty(pg_database_size(datname)) from pg_database;

Example:

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select datname,pg_size_pretty(pg_database_size(datname)) from pg_database;
  datname  | pg_size_pretty
-----------+----------------
 template1 | 61 MB
 template0 | 61 MB
 postgres  | 320 MB
(3 rows)

Querying the Size of a Table and the Size of the Corresponding Index in a Specified Schema

SELECT
    t.tablename,
    indexname,
    c.reltuples AS num_rows,
    pg_size_pretty(pg_relation_size(quote_ident(t.tablename)::text)) AS table_size,
    pg_size_pretty(pg_relation_size(quote_ident(indexrelname)::text)) AS index_size,
    CASE WHEN indisunique THEN 'Y'
       ELSE 'N'
    END AS UNIQUE,
    idx_scan AS number_of_scans,
    idx_tup_read AS tuples_read,
    idx_tup_fetch AS tuples_fetched
FROM pg_tables t
LEFT OUTER JOIN pg_class c ON t.tablename=c.relname
LEFT OUTER JOIN
    ( SELECT c.relname AS ctablename, ipg.relname AS indexname, x.indnatts AS number_of_columns, idx_scan, idx_tup_read, idx_tup_fetch, indexrelname, indisunique FROM pg_index x
           JOIN pg_class c ON c.oid = x.indrelid
           JOIN pg_class ipg ON ipg.oid = x.indexrelid
           JOIN pg_stat_all_indexes psai ON x.indexrelid = psai.indexrelid )
    AS foo
    ON t.tablename = foo.ctablename
WHERE t.schemaname='public'
ORDER BY 1,2;