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The pg_index system catalog table contains part of the information about indexes. The rest is mostly in pg_class.
|indexrelid||oid||pg_class.oid||The OID of the pg_class entry for this index.|
|indrelid||oid||pg_class.oid||The OID of the pg_class entry for the table this index is for.|
|indnatts||int2||The number of columns in the index (duplicates pg_class.relnatts).|
|indisunique||boolean||If true, this is a unique index.|
|indisprimary||boolean||If true, this index represents the primary key of the table. (indisunique should always be true when this is true.)|
|indisclustered||boolean||If true, the table was last clustered on this index via the CLUSTER command.|
|indisvalid||boolean||If true, the index is currently valid for queries. False means the index is possibly incomplete: it must still be modified by INSERT/UPDATE operations, but it cannot safely be used for queries.|
|indkey||int2vector||pg_attribute.attnum||This is an array of indnatts values that indicate which table columns this index indexes. For example a value of 1 3 would mean that the first and the third table columns make up the index key. A zero in this array indicates that the corresponding index attribute is an expression over the table columns, rather than a simple column reference.|
|indclass||oidvector||pg_opclass.oid||For each column in the index key this contains the OID of the operator class to use.|
|indexprs||text||Expression trees (in nodeToString() representation) for index attributes that are not simple column references. This is a list with one element for each zero entry in indkey. NULL if all index attributes are simple references.|
|indpred||text||Expression tree (in nodeToString() representation) for partial index predicate. NULL if not a partial index.|