Oracle patches - some basics and good-to-know information

Oracle Patches can occur very complicated and confusing. Most of the time they are zip files with strange and sometimes inconsistent naming convention.
As these patches are processed by a program called opatch (which also can be obtained as a patch - but it does not follow the same structure as regular patches).

Back to basics: In patches words like product, platform or language are used. They are all IDs and their "data dictionary" can easily be obtained. There are these links:

products	https://updates.oracle.com/Orion/Services/metadata?table=aru_products
releases	https://updates.oracle.com/Orion/Services/metadata?table=aru_releases
platforms	https://updates.oracle.com/Orion/Services/metadata?table=aru_platforms
languages	https://updates.oracle.com/Orion/Services/metadata?table=aru_languages
product_groups	https://updates.oracle.com/Orion/Services/metadata?table=aru_product_groups
product_releases	https://updates.oracle.com/Orion/Services/metadata?table=aru_product_releases
component_releases	https://updates.oracle.com/Orion/Services/metadata?table=aru_component_releases
aru targets	https://updates.oracle.com/Orion/Services/metadata?table=aru_targets
components	https://updates.oracle.com/Orion/Services/components?ctype=product,release,platform,platform_version,wildcard-release,config-group,release-component,code_list
certifications	https://updates.oracle.com/Orion/Services/components?ctype=certification
patch recommendations	https://updates.oracle.com/Orion/Services/recommended_patches

ARU stands for Automated Release Update.

All these files can also obtained in a single zip file: https://updates.oracle.com/download/em_catalog.zip
The name of the zip file also shows it's origin purpose: provide Enterprise Managers recommended patches, download etc. with sufficient information. Some parts like platforms and languages are quite stable, others like releases and  recommendations, change regularly.

recommendations brings uns closer to real patches. it starts with

<results md5_sum="212e2bc3336d1aa3e41074e796201aa9">
  <generated_date in_epoch_ms="1571568490000">2019-10-20 10:48:10</generated_date>
  <patches md5_sum="d58c2ef61a596dd7bdef85b61cd06861">

followed by entries for each patch like
<patch uid="3d4a1929407b7e05430e2fd5bbc18481" has_prereqs="n" has_postreqs="n" is_system_patch="n">

<bug>
        <number>30049606</number>
        <abstract><![CDATA[WINDOWS DB BUNDLE PATCH 12.1.0.2.191015]]></abstract>
      </bug>
      <name>30049606</name>
      <type>Patch</type>
      <psu_bundle>Oracle 12.1.0.2.191015 for Windows</psu_bundle>
      <status>Available</status>
      <access id="m">Open access</access>
      <url>
        <patch_readme host="https://updates.oracle.com"><![CDATA[/Orion/Services/download?type=readme&aru=23166562]]></patch_readme>
        <patch_details><![CDATA[/download/30049606.html]]></patch_details>
      </url>
      <request_id>23166562</request_id>
      <urm_components>
        <qparts ctype_id="201">
          <qpart cid="32615" version="Q5"><![CDATA[Oracle Database - Enterprise Edition]]></qpart>
          <qpart cid="42353" version="Q790"><![CDATA[Oracle Server - Standard Edition]]></qpart>
        </qparts>
        <urm_releases ctype_id="5">
          <urm_release cid="86176" version="12.1.0.2.0"><![CDATA[Oracle Database]]></urm_release>
        </urm_releases>
      </urm_components>
      <product id="9480" bugdb_id="5"><![CDATA[Oracle Database Family]]></product>
      <release id="600000000009300" name="12.1.0.2.0" platform_patch_not_required="Y" cc="Y"><![CDATA[Oracle 12.1.0.2.0]]></release>
      <platform id="912" bugdb_id="912"><![CDATA[Microsoft Windows (32-bit)]]></platform>
      <language id="0" iso_code="EN"><![CDATA[American English]]></language>
      <translations_available>No</translations_available>
      <classification id="185">Security</classification>
      <patch_classification id="185">Security</patch_classification>
      <life_cycle id="175">Recommended</life_cycle>
      <support_level id="E">Extended Support</support_level>
      <entitlements>
        <entitlement code="EXS"/>
      </entitlements>
      <target_types>
        <target_type>
          <aru_target><![CDATA[All Database]]></aru_target>
          <em_tag></em_tag>
        </target_type>
      </target_types>
      <fixes_bugs truncated="no">
        <bug>
          <number>12681464</number>
          <abstract><![CDATA[UPDATE STATEMENT  FAILED - ORA7445 [KGGSLDESTROY] ERROR]]></abstract>
        </bug>
        <bug>
          <number>12799826</number>
          <abstract><![CDATA[LOCK TABLE FOR INDEX FAILS WITH ORA-600 [KKBKAUXBLL0]]]></abstract>
        </bug>
... many more
      </fixes_bugs>
      <size>317964815</size>
      <files>
        <file>
          <name>p30049606_121020_WINNT.zip</name>
          <size>317964815</size>
          <download_url host="https://updates.oracle.com"><![CDATA[/Orion/Services/download/p30049606_121020_WINNT.zip?aru=23166562&patch_file=p30049606_121020_WINNT.zip]]></download_url>
          <digest type="SHA-1">784196EE41085E633F1C10C5F12B47717232C3C5</digest>
          <digest type="SHA-256">849D03DCBB4EDCD2F08474371309470E9D3AE172E4BD7296453F31AEAE40E9D7</digest>
        </file>
      </files>
      <updated_date in_epoch_ms="1571299461000">2019-10-17 08:04:21</updated_date>
      <released_date in_epoch_ms="1570628127000">2019-10-09 13:35:27</released_date>
    </patch>        

Many entries in this XML can be interpreted by the information provided in em_catalog.zip. For each patch there is a download link: https://updates.oracle.com/Orion/Services/download/p30049606_121020_WINNT.zip?aru=23166562&patch_file=p30049606_121020_WINNT.zip


As there are many patches which are not mentioned in recommendations, and also for them there is a link to get an equal XML regarding this patch:
https://updates.oracle.com/Orion/Services/search?bug=<patch_number>

Even the URL suggests it is a search for a BUG, it's a search for a PATCH (as a PATCH can contain fixes for multiple BUGs).

If the same information is required in a HTML form, the URL is
https://updates.oracle.com/Orion/PatchDetails/process_form?patch_num=<patch_number>