OpenLDAP is typically backed by a BerkeleyDB database (bdb). However it's backing datastore is actually pluggable. One of the most intriguing backends is back-sql, which allows LDAP data to be fetched from an ODBC (SQL) datasource. This lets us add a LDAP facade on top of database-backed applications, like JIRA and Confluence.

This guide is specifically for getting OpenLDAP connecting to PostgreSQL on Ubuntu (20.04). Following these instructions should leave you with a Postgres database with users , groups  and members  tables, and corresponding ou=Users  and ou=Groups  LDAP branches.

apt install postgresql-12 postgresql-client-12
su - postgres                                # Switch from root to 'postgres'
createuser -P ldapsql                        # Create 'ldapsql' postgres user. Enter 'ldapsql' for the password. 
createdb -O ldapsql ldapsql                  # Create 'ldapsql' database owned by 'ldapsql'
logout                                       # Switch from 'postgres' back to root

apt install unixodbc odbc-postgresql
cat - <<EOF >> /etc/odbc.ini
[ldapsql]
Description         = Example for OpenLDAP's back-sql
Driver              = PostgreSQL ANSI
Trace               = No
Database            = ldapsql
Servername          = localhost
UserName            = ldapsql
Password            = ldapsql
Port                = 5432
;Protocol            = 6.4
ReadOnly            = No
RowVersioning       = No
ShowSystemTables    = No
ShowOidColumn       = No
FakeOidIndex        = No
ConnSettings        =
EOF

root@openldap2:/# isql -m10 ldapsql <<< 'select * from persons'
+---------------------------------------+
| Connected!                            |
|                                       |
| sql-statement                         |
| help [tablename]                      |
| quit                                  |
|                                       |
+---------------------------------------+
SQL> select * from persons
+-----------+-----------+-----------+-----------+
| id        | name      | surname   | password  |
+-----------+-----------+-----------+-----------+
| 1         | Mitya     | Kovalev   | mit       |
| 2         | Torvlobnor| Puzdoy    |           |
| 3         | Akakiy    | Zinberstei|           |
+-----------+-----------+-----------+-----------+
SQLRowCount returns 3
3 rows fetched

# ldapsearch -x -b 'dc=test,dc=com' -D 'cn=admin,dc=test,dc=com' -w hunter2
# extended LDIF
#
# LDAPv3
# base <dc=test,dc=com> with scope subtree
# filter: (objectclass=*)
# requesting: ALL
#

# test.com
dn: dc=test,dc=com
objectClass: top
objectClass: dcObject
objectClass: organization
o: Test Organization
dc: Test

# admin, test.com
dn: cn=admin,dc=test,dc=com
objectClass: simpleSecurityObject
objectClass: organizationalRole
cn: admin
description: LDAP administrator
userPassword:: e1NTSEF9czRHRHZ4TlNTMWk1eTI0K2cyd3pnTVpFclY4TGpzN2s=

# search result
search: 2
result: 0 Success

# numResponses: 3
# numEntries: 2

cat - <<EOF >> /etc/ldap/slapd.conf

# SQL Backend
database        sql
suffix          "dc=example,dc=com"
rootdn          "cn=admin,dc=example,dc=com"
rootpw          secret
dbname          ldapsql
dbuser          ldapsql
dbpasswd        ldapsql
insentry_stmt   "insert into ldap_entries (id,dn,oc_map_id,parent,keyval) values ((select max(id)+1 from ldap_entries),?,?,?,?)"
upper_func      "upper"
strcast_func    "text"
concat_pattern  "?||?"
#subtree_cond    "ldap_entries.dn LIKE CONCAT('%',?)"
has_ldapinfo_dn_ru      no
EOF
systemctl restart slapd                      # If unsuccessful, 'journalctl -fu slapd &' and try again