前言
今天又遇到了一个问题，如何获取PostgreSQL对象的DDL语句？在Oracle数据库中我们可以使用dbms_metadata.get_ddl函数来处理，那么如何在 PG中实现它呢？
获取DDL的一些方法
1.使用pg_dump来获取ddl
使用pg_dump获取ddl。这个方法最简单。
-s, --schema-only dump only the schema, no data
-t, --table=PATTERN dump the specified table(s) only
[postgres@centos8 ~]$ pg_dump -U postgres -h 192.168.56.119 -d postgres -s -t jobs | egrep -v "^--|^$|^SET"
如下图所示：可以帮助我们显示table的ddl，权限，注释，索引，约束以及触发器。
实际上pg_dump命令行还不错，但需要输入的内容太多了。可考虑使用PL/Python将逻辑封装起来，我们先尝试在操作系统上使用python脚本调用pg_dump。
import subprocess
import re
p_output = subprocess.check_output(["pg_dump", "–-schema-only", "--dbname=postgres", "–-table=jobs", "--username=postgres"],shell=True)
regex_pat=r'(^CREATE TABLE.+?\);$)'
matches=re.findall(regex_pat, p_output.decode("utf-8"),re.DOTALL|re.MULTILINE)
ddl = matches[0]
print(ddl)
下面是我用python编写的脚本，使用的主要是subprocess模块的check_output调用pg_dump命令。接着将返回的字符串与正则表达式匹配，提取创建表的语句。
[postgres@centos8 ~]$ python3 a1.py
CREATE TABLE public.countries (
country_id character(2) NOT NULL,
country_name character varying(40),
region_id bigint
);
目前这个python脚本是写死的，测试功能可用。接下来还需要在postgresql中安装插件plpython3u(python3版本)，创建python语法的函数。
postgres=# CREATE EXTENSION IF NOT EXISTS plpython3u ;
CREATE EXTENSION
CREATE OR REPLACE FUNCTION gel_table_ddl(p_schema VARCHAR,p_database_name VARCHAR,p_table_name VARCHAR)
RETURNS VARCHAR
AS $$
import subprocess
import re
p_output = subprocess.check_output(["pg_dump", "–-schema-only", "--dbname="+p_database_name, "–-table="+p_table_name, "--username="+p_schema],shell=True)
regex_pat=r'(^CREATE TABLE.+?\);$)'
matches=re.findall(regex_pat, p_output.decode("utf-8"),re.DOTALL|re.MULTILINE)
ddl = matches[0]
return ddl
$$ LANGUAGE plpython3u SECURITY DEFINER;
执行结果如图所示
有一点要注意，在我的函数中没有提到密码，密码建议配置. pgpass来实现免密。
2.使用系统函数
PostgreSQL自带了一些函数可以查看 DDL的定义，例如：pg_get_viewdef
、pg_get_constraintdef
、pg_get_functiondef
、pg_get_indexdef
。但奇怪的是它缺少表ddl定义相关函数。
详细信息可以参考：https://www.postgresql.org/docs/current/functions-info.html
不过这问题也难不倒人。在stackoverflow
发现一个脚本，非常好用。
CREATE OR REPLACE FUNCTION tabledef(oid) RETURNS text
LANGUAGE sql STRICT AS $$
/* snatched from https://github.com/filiprem/pg-tools */
WITH attrdef AS (
SELECT
n.nspname,
c.relname,
pg_catalog.array_to_string(c.reloptions || array(select 'toast.' || x from pg_catalog.unnest(tc.reloptions) x), ', ') as relopts,
c.relpersistence,
a.attnum,
a.attname,
pg_catalog.format_type(a.atttypid, a.atttypmod) as atttype,
(SELECT substring(pg_catalog.pg_get_expr(d.adbin, d.adrelid, true) for 128) FROM pg_catalog.pg_attrdef d
WHERE d.adrelid = a.attrelid AND d.adnum = a.attnum AND a.atthasdef) as attdefault,
a.attnotnull,
(SELECT c.collname FROM pg_catalog.pg_collation c, pg_catalog.pg_type t
WHERE c.oid = a.attcollation AND t.oid = a.atttypid AND a.attcollation <> t.typcollation) as attcollation,
a.attidentity,
a.attgenerated
FROM pg_catalog.pg_attribute a
JOIN pg_catalog.pg_class c ON a.attrelid = c.oid
JOIN pg_catalog.pg_namespace n ON c.relnamespace = n.oid
LEFT JOIN pg_catalog.pg_class tc ON (c.reltoastrelid = tc.oid)
WHERE a.attrelid = $1
AND a.attnum > 0
AND NOT a.attisdropped
ORDER BY a.attnum
),
coldef AS (
SELECT
attrdef.nspname,
attrdef.relname,
attrdef.relopts,
attrdef.relpersistence,
pg_catalog.format(
'%I %s%s%s%s%s',
attrdef.attname,
attrdef.atttype,
case when attrdef.attcollation is null then '' else pg_catalog.format(' COLLATE %I', attrdef.attcollation) end,
case when attrdef.attnotnull then ' NOT NULL' else '' end,
case when attrdef.attdefault is null then ''
else case when attrdef.attgenerated = 's' then pg_catalog.format(' GENERATED ALWAYS AS (%s) STORED', attrdef.attdefault)
when attrdef.attgenerated <> '' then ' GENERATED AS NOT_IMPLEMENTED'
else pg_catalog.format(' DEFAULT %s', attrdef.attdefault)
end
end,
case when attrdef.attidentity<>'' then pg_catalog.format(' GENERATED %s AS IDENTITY',
case attrdef.attidentity when 'd' then 'BY DEFAULT' when 'a' then 'ALWAYS' else 'NOT_IMPLEMENTED' end)
else '' end
) as col_create_sql
FROM attrdef
ORDER BY attrdef.attnum
),
tabdef AS (
SELECT
coldef.nspname,
coldef.relname,
coldef.relopts,
coldef.relpersistence,
string_agg(coldef.col_create_sql, E',\n ') as cols_create_sql
FROM coldef
GROUP BY
coldef.nspname, coldef.relname, coldef.relopts, coldef.relpersistence
)
SELECT
format(
'CREATE%s TABLE %I.%I%s%s%s;',
case tabdef.relpersistence when 't' then ' TEMP' when 'u' then ' UNLOGGED' else '' end,
tabdef.nspname,
tabdef.relname,
coalesce(
(SELECT format(E'\n PARTITION OF %I.%I %s\n', pn.nspname, pc.relname,
pg_get_expr(c.relpartbound, c.oid))
FROM pg_class c JOIN pg_inherits i ON c.oid = i.inhrelid
JOIN pg_class pc ON pc.oid = i.inhparent
JOIN pg_namespace pn ON pn.oid = pc.relnamespace
WHERE c.oid = $1),
format(E' (\n %s\n)', tabdef.cols_create_sql)
),
case when tabdef.relopts <> '' then format(' WITH (%s)', tabdef.relopts) else '' end,
coalesce(E'\nPARTITION BY '||pg_get_partkeydef($1), '')
) as table_create_sql
FROM tabdef
$$;
执行函数结果如图所示：
若要获取索引，直接使用系统自带的pg_get_indexdef函数。
3.使用pgddl插件
最后来介绍的是一款插件，知道和使用它的人不多，看了一下作者写的 roadmap，还是很有动力的。我测试了一下，在PostgreSQL 13版本上也可以使用，就列出来作为一种选择。
要使用root用户安装此插件，并在安装时设置好环境变量。
export PGHOME=/data/postgresql/pgsql
export PGDATA=/data/postgresql/pgdata
[root@centos8 pgddl]# make
[root@centos8 pgddl]# make install
/usr/bin/mkdir -p '/data/postgresql/pgsql/share/extension'
/usr/bin/mkdir -p '/data/postgresql/pgsql/share/extension'
/usr/bin/install -c -m 644 .//ddlx.control '/data/postgresql/pgsql/share/extension/'
/usr/bin/install -c -m 644 ddlx--0.17.sql '/data/postgresql/pgsql/share/extension/'
在完成安装之后，进入PostgreSQL数据库创建扩展。
postgres=# CREATE EXTENSION ddlx;
CREATE EXTENSION
执行结果如图所示，像pg_dump一样，您也可以显示这个表中所有的相关信息。
如果想要单独显示其他的对象，只需输入一个名字，它会自动将该名字的ddl全部列出来。
参考链接
1.https://stackoverflow.com/questions/1846542/postgresql-get-table-definition-pg-get-tabledef
2.https://github.com/lacanoid/pgddl
3.https://proboscid.wordpress.com/2013/09/06/extracting-create-table-ddl-from-postgresql/
4.https://www.postgresql.org/docs/current/functions-info.html
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