pg_index

The pg_index system catalog table contains part of the information about indexes. The rest is mostly in pg_class_internal.

Table 1. pg_catalog.pg_index

column	type	references	description
indexrelid	oid	pg_class_internal_internal.oid	The OID of the pg_class_internal entry for this index.
indrelid	oid	pg_class_internal.oid	The OID of the pg_class_internal entry for the table this index is for.
indnatts	smallint		The number of columns in the index (duplicates pg_class_internal.relnatts).
indisunique	boolean		If true, this is a unique index.
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 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.