{"id":46636,"date":"2026-08-26T10:54:51","date_gmt":"2026-08-26T08:54:51","guid":{"rendered":"https:\/\/www.dbi-services.com\/blog\/?p=46636"},"modified":"2026-08-26T10:54:54","modified_gmt":"2026-08-26T08:54:54","slug":"fixing-ora-00904-on-oracle-hidden-columns-during-ssma-data-migration","status":"publish","type":"post","link":"https:\/\/www.dbi-services.com\/blog\/fixing-ora-00904-on-oracle-hidden-columns-during-ssma-data-migration\/","title":{"rendered":"Fixing ORA-00904 on Oracle hidden columns during SSMA data migration"},"content":{"rendered":"\n<p class=\"wp-block-paragraph\"><a href=\"https:\/\/learn.microsoft.com\/en-us\/sql\/ssma\/sql-server-migration-assistant?view=sql-server-ver17\">SSMA <\/a>(SQL Server Migration Assistant) handles the whole Oracle-to-SQL Server move: it reads the source data dictionary, converts the schema, then generates a SELECT per table to pull the rows across. That last step is where this story goes wrong.<\/p>\n\n\n\n<h2 id=\"h-the-problem\" class=\"wp-block-heading\">The problem<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Migrating ~5,600 Oracle tables to SQL Server with SSMA. Most load fine; ~100 tables fail <strong>Migrate Data<\/strong> with the same error:<\/p>\n\n\n<div class=\"wp-block-syntaxhighlighter-code \"><pre class=\"brush: plain; title: ; notranslate\" title=\"\">\nERROR &#x5B;42S22] &#x5B;Oracle]&#x5B;ODBC]&#x5B;Ora]ORA-00904:\n&quot;SYS_C00004_21081414:28:22$&quot;: invalid identifier\n<\/pre><\/div>\n\n\n<p class=\"wp-block-paragraph\">The named column exists in no DDL anyone wrote. The <code>[Ora]<\/code> prefix says Oracle itself is rejecting the query: SSMA built an extraction SELECT naming a column Oracle refuses to resolve. Tellingly, <code>SELECT *<\/code> and <code>COUNT(*)<\/code> run fine against the same table, whatever this column is, Oracle is happy to ignore it, but not to be asked for it by name.<\/p>\n\n\n\n<h2 id=\"h-where-the-column-actually-comes-from\" class=\"wp-block-heading\">Where the column actually comes from<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">The name is the giveaway: <code>SYS_C00004_21081414:28:22$<\/code> is what Oracle calls a column after <code>ALTER TABLE \u2026 SET UNUSED COLUMN<\/code>.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Oracle offers two ways to get rid of a column: a logical delete and a physical one. The physical delete (<code>ALTER TABLE \u2026 DROP COLUMN<\/code>) is the honest one, but on a large table it is very time- and resource-consuming. That&#8217;s why people reach for the logical delete instead:<\/p>\n\n\n<div class=\"wp-block-syntaxhighlighter-code \"><pre class=\"brush: sql; title: ; notranslate\" title=\"\">\nALTER TABLE table_name SET UNUSED (column_name);\n<\/pre><\/div>\n\n\n<p class=\"wp-block-paragraph\">That statement is metadata-only and instant. The column immediately stops being visible to users, and the physical removal is deferred to whenever there is time for it (see <a href=\"https:\/\/oracle-base.com\/articles\/8i\/dropping-columns\" data-type=\"link\" data-id=\"https:\/\/oracle-base.com\/articles\/8i\/dropping-columns\">Oracle Documentation<\/a>):<\/p>\n\n\n<div class=\"wp-block-syntaxhighlighter-code \"><pre class=\"brush: sql; title: ; notranslate\" title=\"\">\nALTER TABLE table_name DROP UNUSED COLUMNS;\n-- on large tables, cap undo growth by checkpointing every N rows:\nALTER TABLE table_name DROP UNUSED COLUMNS CHECKPOINT 250;\n<\/pre><\/div>\n\n\n<p class=\"wp-block-paragraph\">To free the original name for reuse, Oracle renames the column to <code>SYS_C&lt;internal column number&gt;_&lt;YYMMDDHH24:MI:SS&gt;$<\/code>, sets <code>USER_GENERATED<\/code> to NO, <code>HIDDEN_COLUMN<\/code> to YES and releases its <code>COLUMN_ID<\/code>. So the timestamp is not when the column was added, it is the second someone ran <code>SET UNUSED<\/code>. Ours says 14 August 2021, 14:28:22.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">That also explains the error pattern. The operation is one-way and the column is unreadable by design, so naming it gets you ORA-00904 \u00abinvalid identifier\u00bb. <code>SELECT *<\/code> and <code>COUNT(*)<\/code> keep working because the column no longer has a <code>COLUMN_ID<\/code> and is simply excluded from the star. SSMA, however, lists it and builds an explicit column list Oracle then refuses.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Not to be confused with <code>SYS_NC\u2026$<\/code>.<\/strong> Those are a different animal: virtual columns backing a function-based index or extended statistics. <\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>Bottom line:<\/strong> returning NULL costs you nothing. This is a column its owner already decided to delete, holding data Oracle itself will no longer hand out. There is no information left to lose.<\/p>\n\n\n\n<h2 id=\"h-what-doesn-t-work-for-the-migration\" class=\"wp-block-heading\">What doesn&#8217;t work for the migration<\/h2>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>SSMA setting <\/strong><code>Ignore hidden system columns = Yes<\/code> was not making any effect on this use case<\/li>\n\n\n\n<li><strong>Dropping the column on SQL Server<\/strong> resolves nothing because the error is on the source SELECT, unaffected.<\/li>\n\n\n\n<li><strong>Dropping it on Oracle<\/strong> could not be done in our scenario because the source is frozen; DDL not allowed.<\/li>\n\n\n\n<li><strong>Custom select, column removed or bare <code>NULL<\/code><\/strong>: SSMA still expects the name in its mapping and fails with &#8220;key not present&#8221; or &#8220;does not match up&#8221; before the query ever reaches Oracle.<\/li>\n<\/ul>\n\n\n\n<h2 id=\"h-find-them-all-first\" class=\"wp-block-heading\">Find them all first<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Before editing anything, get the full list. Discovering the affected tables one failed migration at a time is a waste of an afternoon because the data dictionary already knows.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The reason the columns are findable at all is an asymmetry between two views: an unused column is gone from <code>ALL_TAB_COLUMNS<\/code>, but still listed in <code>ALL_TAB_COLS<\/code> with <code>HIDDEN_COLUMN = 'YES'<\/code>. That second view is what you query:<\/p>\n\n\n<div class=\"wp-block-syntaxhighlighter-code \"><pre class=\"brush: sql; title: ; notranslate\" title=\"\">\nSELECT owner,\ntable_name,\ncolumn_name,\ndata_type,\ninternal_column_id,\nTO_DATE(REGEXP_SUBSTR(column_name, &#039;\\d{8}:\\d{2}:\\d{2}&#039;),\n&#039;YYMMDDHH24:MI:SS&#039;) AS set_unused_at\nFROM dba_tab_cols\nWHERE hidden_column = &#039;YES&#039;\nAND user_generated = &#039;NO&#039;\nAND REGEXP_LIKE(column_name, &#039;^SYS_C\\d+_\\d{8}:\\d{2}:\\d{2}\\$$&#039;)\n-- AND owner = &#039;&lt;SCHEMA_NAME&gt;&#039;\nORDER BY owner, table_name, internal_column_id;\n<\/pre><\/div>\n\n\n<p class=\"wp-block-paragraph\">The regex is deliberately strict: it matches only the <code>SET UNUSED<\/code> naming pattern, so virtual columns and other system-generated names stay out of the result. <\/p>\n\n\n\n<p class=\"wp-block-paragraph\">One more view is worth a look, as a cross-check:<\/p>\n\n\n<div class=\"wp-block-syntaxhighlighter-code \"><pre class=\"brush: sql; title: ; notranslate\" title=\"\">\nSELECT owner, table_name, count AS unused_columns\nFROM   dba_unused_col_tabs\n--WHERE owner = &#039;&lt;SCHEMA_NAME&gt;&#039;\nORDER  BY count DESC, table_name;\n<\/pre><\/div>\n\n\n<p class=\"wp-block-paragraph\"><code>DBA_UNUSED_COL_TABS<\/code> gives the number of unused columns per table. Sorting by that number puts the dangerous tables first: those with two or three hidden columns are the ones where you&#8217;ll forget a line in the custom select and be back at square one.<\/p>\n\n\n\n<h2 id=\"h-the-fix\" class=\"wp-block-heading\">The fix<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Keep the hidden column&#8217;s name as an <strong>alias<\/strong>, but return a literal NULL instead of reading it. SSMA&#8217;s mapping finds the name (no &#8220;key not present&#8221;); Oracle never resolves the real column (no ORA-00904).<\/p>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Tools \u2192 Project Settings \u2192 General \u2192 Migration<\/strong> \u2192 enable <strong>Extended data migration options<\/strong>.<\/li>\n<\/ul>\n\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Data Migration Settings<\/strong> tab \u2192 tick <strong>Use custom select<\/strong> \u2192 replace each hidden-column line with:<\/li>\n<\/ul>\n\n\n\n<ol class=\"wp-block-list\"><\/ol>\n\n\n<div class=\"wp-block-syntaxhighlighter-code \"><pre class=\"brush: sql; title: ; notranslate\" title=\"\">\n SELECT ...   \n    TO_CHAR(&quot;&lt;COLUMN_NAME&gt;&quot;, &#039;TM&#039;, &#039;NLS_NUMERIC_CHARACTERS = &#039;&#039;.,&#039;&#039;&#039;) as &quot;&lt;COLUMN_NAME&gt;&quot;,\n    NULL as &quot;SYS_C00004_21081414:28:22$&quot;\nfrom &lt;OWNER&gt;.&lt;TABLE_NAME&gt; t\n<\/pre><\/div>\n\n\n<ul class=\"wp-block-list\">\n<li><strong>Migrate Data<\/strong> \u2192 100%. Drop the NULL-filled column(s) on SQL Server in post-migration cleanup.<\/li>\n<\/ul>\n\n\n\n<ol class=\"wp-block-list\"><\/ol>\n\n\n\n<h2 id=\"h-takeaway\" class=\"wp-block-heading\">Takeaway<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\"><code>SYS_C\u2026$<\/code> is not an exotic Oracle feature, it&#8217;s an ordinary column someone deleted years ago, logically. Oracle keeps the name on file; SSMA finds it, insists on naming it, and Oracle refuses to hand it over. Aliasing a NULL satisfies both, then you drop the column on the target. No source DDL, no external tooling, everything inside SSMA, behind a project setting that&#8217;s hidden by default.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>SSMA fails with ORA-00904 on Oracle SET UNUSED columns. Fix it with a custom select aliasing NULL, no source DDL required.<\/p>\n","protected":false},"author":157,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":"","_members_access_role":[],"_members_access_error":""},"categories":[198,59,99],"tags":[96,51,4198],"type_dbi":[],"class_list":["post-46636","post","type-post","status-publish","format-standard","hentry","category-database-management","category-oracle","category-sql-server","tag-oracle","tag-sql-server","tag-ssma"],"acf":[],"yoast_head":"<!-- This site is optimized with the Yoast SEO Premium plugin v28.2 (Yoast SEO v28.3) - https:\/\/yoast.com\/product\/yoast-seo-premium-wordpress\/ -->\n<title>Fixing ORA-00904 on Oracle hidden columns during SSMA data migration - dbi Blog<\/title>\n<meta name=\"description\" content=\"SSMA fails with ORA-00904 on Oracle SET UNUSED columns. Fix it with a custom select aliasing NULL, no source DDL required.\" \/>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/www.dbi-services.com\/blog\/fixing-ora-00904-on-oracle-hidden-columns-during-ssma-data-migration\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Fixing ORA-00904 on Oracle hidden columns during SSMA data migration\" \/>\n<meta property=\"og:description\" content=\"SSMA fails with ORA-00904 on Oracle SET UNUSED columns. 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