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path: root/src/backend/parser/parse_utilcmd.c
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2017-03-28Cast result of copyObject() to correct typePeter Eisentraut
copyObject() is declared to return void *, which allows easily assigning the result independent of the input, but it loses all type checking. If the compiler supports typeof or something similar, cast the result to the input type. This creates a greater amount of type safety. In some cases, where the result is assigned to a generic type such as Node * or Expr *, new casts are now necessary, but in general casts are now unnecessary in the normal case and indicate that something unusual is happening. Reviewed-by: Mark Dilger <hornschnorter@gmail.com>
2017-03-06Remove objname/objargs split for referring to objectsPeter Eisentraut
In simpler times, it might have worked to refer to all kinds of objects by a list of name components and an optional argument list. But this doesn't work for all objects, which has resulted in a collection of hacks to place various other nodes types into these fields, which have to be unpacked at the other end. This makes it also weird to represent lists of such things in the grammar, because they would have to be lists of singleton lists, to make the unpacking work consistently. The other problem is that keeping separate name and args fields makes it awkward to deal with lists of functions. Change that by dropping the objargs field and have objname, renamed to object, be a generic Node, which can then be flexibly assigned and managed using the normal Node mechanisms. In many cases it will still be a List of names, in some cases it will be a string Value, for types it will be the existing Typename, for functions it will now use the existing ObjectWithArgs node type. Some of the more obscure object types still use somewhat arbitrary nested lists. Reviewed-by: Jim Nasby <Jim.Nasby@BlueTreble.com> Reviewed-by: Michael Paquier <michael.paquier@gmail.com>
2017-02-21Make more use of castNode()Peter Eisentraut
2017-02-16Avoid crash in ALTER TABLE not_partitioned DETACH PARTITION.Robert Haas
Amit Langote, reviewed and slightly changed by me.
2017-02-10Add CREATE SEQUENCE AS <data type> clausePeter Eisentraut
This stores a data type, required to be an integer type, with the sequence. The sequences min and max values default to the range supported by the type, and they cannot be set to values exceeding that range. The internal implementation of the sequence is not affected. Change the serial types to create sequences of the appropriate type. This makes sure that the min and max values of the sequence for a serial column match the range of values supported by the table column. So the sequence can no longer overflow the table column. This also makes monitoring for sequence exhaustion/wraparound easier, which currently requires various contortions to cross-reference the sequences with the table columns they are used with. This commit also effectively reverts the pg_sequence column reordering in f3b421da5f4addc95812b9db05a24972b8fd9739, because the new seqtypid column allows us to fill the hole in the struct and create a more natural overall column ordering. Reviewed-by: Steve Singer <steve@ssinger.info> Reviewed-by: Michael Paquier <michael.paquier@gmail.com>
2017-02-06Fix typos in comments.Heikki Linnakangas
Backpatch to all supported versions, where applicable, to make backpatching of future fixes go more smoothly. Josh Soref Discussion: https://www.postgresql.org/message-id/CACZqfqCf+5qRztLPgmmosr-B0Ye4srWzzw_mo4c_8_B_mtjmJQ@mail.gmail.com
2017-01-03Update copyright via script for 2017Bruce Momjian
2016-12-22Fix CREATE TABLE ... LIKE ... WITH OIDS.Tom Lane
Having a WITH OIDS specification should result in the creation of an OID column, but commit b943f502b broke that in the case that there were LIKE tables without OIDS. Commentary in that patch makes it look like this was intentional, but if so it was based on a faulty reading of what inheritance does: the parent tables can add an OID column, but they can't subtract one. AFAICS, the behavior ought to be that you get an OID column if any of the inherited tables, LIKE tables, or WITH clause ask for one. Also, revert that patch's unnecessary split of transformCreateStmt's loop over the tableElts list into two passes. That seems to have been based on a misunderstanding as well: we already have two-pass processing here, we don't need three passes. Per bug #14474 from Jeff Dafoe. Back-patch to 9.6 where the misbehavior was introduced. Report: https://postgr.es/m/20161222145304.25620.47445@wrigleys.postgresql.org
2016-12-07Implement table partitioning.Robert Haas
Table partitioning is like table inheritance and reuses much of the existing infrastructure, but there are some important differences. The parent is called a partitioned table and is always empty; it may not have indexes or non-inherited constraints, since those make no sense for a relation with no data of its own. The children are called partitions and contain all of the actual data. Each partition has an implicit partitioning constraint. Multiple inheritance is not allowed, and partitioning and inheritance can't be mixed. Partitions can't have extra columns and may not allow nulls unless the parent does. Tuples inserted into the parent are automatically routed to the correct partition, so tuple-routing ON INSERT triggers are not needed. Tuple routing isn't yet supported for partitions which are foreign tables, and it doesn't handle updates that cross partition boundaries. Currently, tables can be range-partitioned or list-partitioned. List partitioning is limited to a single column, but range partitioning can involve multiple columns. A partitioning "column" can be an expression. Because table partitioning is less general than table inheritance, it is hoped that it will be easier to reason about properties of partitions, and therefore that this will serve as a better foundation for a variety of possible optimizations, including query planner optimizations. The tuple routing based which this patch does based on the implicit partitioning constraints is an example of this, but it seems likely that many other useful optimizations are also possible. Amit Langote, reviewed and tested by Robert Haas, Ashutosh Bapat, Amit Kapila, Rajkumar Raghuwanshi, Corey Huinker, Jaime Casanova, Rushabh Lathia, Erik Rijkers, among others. Minor revisions by me.
2016-09-13Improve parser's and planner's handling of set-returning functions.Tom Lane
Teach the parser to reject misplaced set-returning functions during parse analysis using p_expr_kind, in much the same way as we do for aggregates and window functions (cf commit eaccfded9). While this isn't complete (it misses nesting-based restrictions), it's much better than the previous error reporting for such cases, and it allows elimination of assorted ad-hoc expression_returns_set() error checks. We could add nesting checks later if it seems important to catch all cases at parse time. There is one case the parser will now throw error for although previous versions allowed it, which is SRFs in the tlist of an UPDATE. That never behaved sensibly (since it's ill-defined which generated row should be used to perform the update) and it's hard to see why it should not be treated as an error. It's a release-note-worthy change though. Also, add a new Query field hasTargetSRFs reporting whether there are any SRFs in the targetlist (including GROUP BY/ORDER BY expressions). The parser can now set that basically for free during parse analysis, and we can use it in a number of places to avoid expression_returns_set searches. (There will be more such checks soon.) In some places, this allows decontorting the logic since it's no longer expensive to check for SRFs in the tlist --- so I made the checks parallel to the handling of hasAggs/hasWindowFuncs wherever it seemed appropriate. catversion bump because adding a Query field changes stored rules. Andres Freund and Tom Lane Discussion: <24639.1473782855@sss.pgh.pa.us>
2016-09-06Add location field to DefElemPeter Eisentraut
Add a location field to the DefElem struct, used to parse many utility commands. Update various error messages to supply error position information. To propogate the error position information in a more systematic way, create a ParseState in standard_ProcessUtility() and pass that to interested functions implementing the utility commands. This seems better than passing the query string and then reassembling a parse state ad hoc, which violates the encapsulation of the ParseState type. Reviewed-by: Pavel Stehule <pavel.stehule@gmail.com>
2016-07-28Improve documentation about CREATE TABLE ... LIKE.Tom Lane
The docs failed to explain that LIKE INCLUDING INDEXES would not preserve the names of indexes and associated constraints. Also, it wasn't mentioned that EXCLUDE constraints would be copied by this option. The latter oversight seems enough of a documentation bug to justify back-patching. In passing, do some minor copy-editing in the same area, and add an entry for LIKE under "Compatibility", since it's not exactly a faithful implementation of the standard's feature. Discussion: <20160728151154.AABE64016B@smtp.hushmail.com>
2016-06-09pgindent run for 9.6Robert Haas
2016-04-08Revert CREATE INDEX ... INCLUDING ...Teodor Sigaev
It's not ready yet, revert two commits 690c543550b0d2852060c18d270cdb534d339d9a - unstable test output 386e3d7609c49505e079c40c65919d99feb82505 - patch itself
2016-04-08CREATE INDEX ... INCLUDING (column[, ...])Teodor Sigaev
Now indexes (but only B-tree for now) can contain "extra" column(s) which doesn't participate in index structure, they are just stored in leaf tuples. It allows to use index only scan by using single index instead of two or more indexes. Author: Anastasia Lubennikova with minor editorializing by me Reviewers: David Rowley, Peter Geoghegan, Jeff Janes
2016-03-23Support CREATE ACCESS METHODAlvaro Herrera
This enables external code to create access methods. This is useful so that extensions can add their own access methods which can be formally tracked for dependencies, so that DROP operates correctly. Also, having explicit support makes pg_dump work correctly. Currently only index AMs are supported, but we expect different types to be added in the future. Authors: Alexander Korotkov, Petr Jelínek Reviewed-By: Teodor Sigaev, Petr Jelínek, Jim Nasby Commitfest-URL: https://commitfest.postgresql.org/9/353/ Discussion: https://www.postgresql.org/message-id/CAPpHfdsXwZmojm6Dx+TJnpYk27kT4o7Ri6X_4OSWcByu1Rm+VA@mail.gmail.com
2016-02-11Move pg_constraint.h function declarations to new file pg_constraint_fn.h.Tom Lane
A pending patch requires exporting a function returning Bitmapset from catalog/pg_constraint.c. As things stand, that would mean including nodes/bitmapset.h in pg_constraint.h, which might be hazardous for the client-side includability of that header. It's not entirely clear whether any client-side code needs to include pg_constraint.h, but it seems prudent to assume that there is some such code somewhere. Therefore, split off the function definitions into a new file pg_constraint_fn.h, similarly to what we've done for some other catalog header files.
2016-01-17Restructure index access method API to hide most of it at the C level.Tom Lane
This patch reduces pg_am to just two columns, a name and a handler function. All the data formerly obtained from pg_am is now provided in a C struct returned by the handler function. This is similar to the designs we've adopted for FDWs and tablesample methods. There are multiple advantages. For one, the index AM's support functions are now simple C functions, making them faster to call and much less error-prone, since the C compiler can now check function signatures. For another, this will make it far more practical to define index access methods in installable extensions. A disadvantage is that SQL-level code can no longer see attributes of index AMs; in particular, some of the crosschecks in the opr_sanity regression test are no longer possible from SQL. We've addressed that by adding a facility for the index AM to perform such checks instead. (Much more could be done in that line, but for now we're content if the amvalidate functions more or less replace what opr_sanity used to do.) We might also want to expose some sort of reporting functionality, but this patch doesn't do that. Alexander Korotkov, reviewed by Petr Jelínek, and rather heavily editorialized on by me.
2016-01-02Update copyright for 2016Bruce Momjian
Backpatch certain files through 9.1
2015-12-16Mark CHECK constraints declared NOT VALID valid if created with table.Robert Haas
FOREIGN KEY constraints have behaved this way for a long time, but for some reason the behavior of CHECK constraints has been inconsistent up until now. Amit Langote and Amul Sul, with assorted tweaks by me.
2015-10-22Fix typos in comments.Robert Haas
CharSyam
2015-10-05Have CREATE TABLE LIKE add OID column if any LIKEd table has oneBruce Momjian
Also, process constraints for LIKEd tables at the end so an OID column can be referenced in a constraint. Report by Tom Lane
2015-05-23pgindent run for 9.5Bruce Momjian
2015-04-28Fix another test for RELKIND_RELATION that should allow foreign tables now.Tom Lane
I thought I'd gone through all of these before, but a fresh review found this one too. (Perhaps it would be better to just delete this test and let the failure occur later, but for the moment I'll preserve the logic.) The case that this was rejecting is like CREATE FOREIGN TABLE ft (f1 int ...) ...; CREATE TABLE c1 (UNIQUE(f1)) INHERITS(ft);
2015-04-25Add comments warning against generalizing default_with_oids.Tom Lane
pg_dump has historically assumed that default_with_oids affects only plain tables and not other relkinds. Conceivably we could make it apply to some newly invented relkind if we did so from the get-go, but changing the behavior for existing object types will break existing dump scripts. Add code comments warning about this interaction. Also, make sure that default_with_oids doesn't cause parse_utilcmd.c to think that CREATE FOREIGN TABLE will create an OID column. I think this is only a latent bug right now, since we don't allow UNIQUE/PKEY constraints in CREATE FOREIGN TABLE, but it's better to be consistent and future-proof.
2015-04-25Revert: Honor OID status of CREATE LIKE'd tablesBruce Momjian
Reverts d992f8a8961c09ec219373ffe2b5e6473febd065 Report by Tom Lane
2015-04-20Honor OID status of CREATE LIKE'd tablesBruce Momjian
Previously, tables created by CREATE LIKE never had OIDs. Report by Tom Lane
2015-04-03Transform ALTER TABLE/SET TYPE/USING expr during parse analysisAlvaro Herrera
This lets later stages have access to the transformed expression; in particular it allows DDL-deparsing code during event triggers to pass the transformed expression to ruleutils.c, so that the complete command can be deparsed. This shuffles the timing of the transform calls a bit: previously, nothing was transformed during parse analysis, and only the RELKIND_RELATION case was being handled during execution. After this patch, all expressions are transformed during parse analysis (including those for relkinds other than RELATION), and the error for other relation kinds is thrown only during execution. So we do more work than before to reject some bogus cases. That seems acceptable.
2015-03-18Setup cursor position for schema-qualified elementsAlvaro Herrera
This makes any errors thrown while looking up such schemas report the position of the error. Author: Ryan Kelly Reviewed by: Jeevan Chalke, Tom Lane
2015-03-09Allow CURRENT/SESSION_USER to be used in certain commandsAlvaro Herrera
Commands such as ALTER USER, ALTER GROUP, ALTER ROLE, GRANT, and the various ALTER OBJECT / OWNER TO, as well as ad-hoc clauses related to roles such as the AUTHORIZATION clause of CREATE SCHEMA, the FOR clause of CREATE USER MAPPING, and the FOR ROLE clause of ALTER DEFAULT PRIVILEGES can now take the keywords CURRENT_USER and SESSION_USER as user specifiers in place of an explicit user name. This commit also fixes some quite ugly handling of special standards- mandated syntax in CREATE USER MAPPING, which in particular would fail to work in presence of a role named "current_user". The special role specifiers PUBLIC and NONE also have more consistent handling now. Also take the opportunity to add location tracking to user specifiers. Authors: Kyotaro Horiguchi. Heavily reworked by Álvaro Herrera. Reviewed by: Rushabh Lathia, Adam Brightwell, Marti Raudsepp.
2015-02-22Get rid of multiple applications of transformExpr() to the same tree.Tom Lane
transformExpr() has for many years had provisions to do nothing when applied to an already-transformed expression tree. However, this was always ugly and of dubious reliability, so we'd be much better off without it. The primary historical reason for it was that gram.y sometimes returned multiple links to the same subexpression, which is no longer true as of my BETWEEN fixes. We'd also grown some lazy hacks in CREATE TABLE LIKE (failing to distinguish between raw and already-transformed index specifications) and one or two other places. This patch removes the need for and support for re-transforming already transformed expressions. The index case is dealt with by adding a flag to struct IndexStmt to indicate that it's already been transformed; which has some benefit anyway in that tablecmds.c can now Assert that transformation has happened rather than just assuming. The other main reason was some rather sloppy code for array type coercion, which can be fixed (and its performance improved too) by refactoring. I did leave transformJoinUsingClause() still constructing expressions containing untransformed operator nodes being applied to Vars, so that transformExpr() still has to allow Var inputs. But that's a much narrower, and safer, special case than before, since Vars will never appear in a raw parse tree, and they don't have any substructure to worry about. In passing fix some oversights in the patch that added CREATE INDEX IF NOT EXISTS (missing processing of IndexStmt.if_not_exists). These appear relatively harmless, but still sloppy coding practice.
2015-02-03Fix typo in comment.Heikki Linnakangas
Amit Langote
2015-01-06Update copyright for 2015Bruce Momjian
Backpatch certain files through 9.0
2014-12-23get_object_address: separate domain constraints from table constraintsAlvaro Herrera
Apart from enabling comments on domain constraints, this enables a future project to replicate object dropping to remote servers: with the current mechanism there's no way to distinguish between the two types of constraints, so there's no way to know what to drop. Also added support for the domain constraint comments in psql's \dd and pg_dump. Catalog version bumped due to the change in ObjectType enum.
2014-12-17Allow CHECK constraints to be placed on foreign tables.Tom Lane
As with NOT NULL constraints, we consider that such constraints are merely reports of constraints that are being enforced by the remote server (or other underlying storage mechanism). Their only real use is to allow planner optimizations, for example in constraint-exclusion checks. Thus, the code changes here amount to little more than removal of the error that was formerly thrown for applying CHECK to a foreign table. (In passing, do a bit of cleanup of the ALTER FOREIGN TABLE reference page, which had accumulated some weird decisions about ordering etc.) Shigeru Hanada and Etsuro Fujita, reviewed by Kyotaro Horiguchi and Ashutosh Bapat.
2014-05-06pgindent run for 9.4Bruce Momjian
This includes removing tabs after periods in C comments, which was applied to back branches, so this change should not effect backpatching.
2014-02-17Avoid repeated name lookups during table and index DDL.Robert Haas
If the name lookups come to different conclusions due to concurrent activity, we might perform some parts of the DDL on a different table than other parts. At least in the case of CREATE INDEX, this can be used to cause the permissions checks to be performed against a different table than the index creation, allowing for a privilege escalation attack. This changes the calling convention for DefineIndex, CreateTrigger, transformIndexStmt, transformAlterTableStmt, CheckIndexCompatible (in 9.2 and newer), and AlterTable (in 9.1 and older). In addition, CheckRelationOwnership is removed in 9.2 and newer and the calling convention is changed in older branches. A field has also been added to the Constraint node (FkConstraint in 8.4). Third-party code calling these functions or using the Constraint node will require updating. Report by Andres Freund. Patch by Robert Haas and Andres Freund, reviewed by Tom Lane. Security: CVE-2014-0062
2014-01-07Update copyright for 2014Bruce Momjian
Update all files in head, and files COPYRIGHT and legal.sgml in all back branches.
2013-11-21Support multi-argument UNNEST(), and TABLE() syntax for multiple functions.Tom Lane
This patch adds the ability to write TABLE( function1(), function2(), ...) as a single FROM-clause entry. The result is the concatenation of the first row from each function, followed by the second row from each function, etc; with NULLs inserted if any function produces fewer rows than others. This is believed to be a much more useful behavior than what Postgres currently does with multiple SRFs in a SELECT list. This syntax also provides a reasonable way to combine use of column definition lists with WITH ORDINALITY: put the column definition list inside TABLE(), where it's clear that it doesn't control the ordinality column as well. Also implement SQL-compliant multiple-argument UNNEST(), by turning UNNEST(a,b,c) into TABLE(unnest(a), unnest(b), unnest(c)). The SQL standard specifies TABLE() with only a single function, not multiple functions, and it seems to require an implicit UNNEST() which is not what this patch does. There may be something wrong with that reading of the spec, though, because if it's right then the spec's TABLE() is just a pointless alternative spelling of UNNEST(). After further review of that, we might choose to adopt a different syntax for what this patch does, but in any case this functionality seems clearly worthwhile. Andrew Gierth, reviewed by Zoltán Böszörményi and Heikki Linnakangas, and significantly revised by me
2013-08-07Message style improvementsPeter Eisentraut
2013-07-05Update messages, comments and documentation for materialized views.Noah Misch
All instances of the verbiage lagging the code. Back-patch to 9.3, where materialized views were introduced.
2013-07-01Add a convenience routine makeFuncCall to reduce duplication.Robert Haas
David Fetter and Andrew Gierth, reviewed by Jeevan Chalke
2013-05-29pgindent run for release 9.3Bruce Momjian
This is the first run of the Perl-based pgindent script. Also update pgindent instructions.
2013-04-20Clean up references to SQL92Peter Eisentraut
In most cases, these were just references to the SQL standard in general. In a few cases, a contrast was made between SQL92 and later standards -- those have been kept unchanged.
2013-04-12Clean up the mess around EXPLAIN and materialized views.Tom Lane
Revert the matview-related changes in explain.c's API, as per recent complaint from Robert Haas. The reason for these appears to have been principally some ill-considered choices around having intorel_startup do what ought to be parse-time checking, plus a poor arrangement for passing it the view parsetree it needs to store into pg_rewrite when creating a materialized view. Do the latter by having parse analysis stick a copy into the IntoClause, instead of doing it at runtime. (On the whole, I seriously question the choice to represent CREATE MATERIALIZED VIEW as a variant of SELECT INTO/CREATE TABLE AS, because that means injecting even more complexity into what was already a horrid legacy kluge. However, I didn't go so far as to rethink that choice ... yet.) I also moved several error checks into matview parse analysis, and made the check for external Params in a matview more accurate. In passing, clean things up a bit more around interpretOidsOption(), and fix things so that we can use that to force no-oids for views, sequences, etc, thereby eliminating the need to cons up "oids = false" options when creating them. catversion bump due to change in IntoClause. (I wonder though if we really need readfuncs/outfuncs support for IntoClause anymore.)
2013-03-22Fix problems with incomplete attempt to prohibit OIDS with MVs.Kevin Grittner
Problem with assertion failure in restoring from pg_dump output reported by Joachim Wieland. Review and suggestions by Tom Lane and Robert Haas.
2013-03-12Allow default expressions to be attached to columns of foreign tables.Tom Lane
There's still some discussion about exactly how postgres_fdw ought to handle this case, but there seems no debate that we want to allow defaults to be used for inserts into foreign tables. So remove the core-code restrictions that prevented it. While at it, get rid of the special grammar productions for CREATE FOREIGN TABLE, and instead add explicit FEATURE_NOT_SUPPORTED error checks for the disallowed cases. This makes the grammar a shade smaller, and more importantly results in much more intelligible error messages for unsupported cases. It's also one less thing to fix if we ever start supporting constraints on foreign tables.
2013-03-03Add a materialized view relations.Kevin Grittner
A materialized view has a rule just like a view and a heap and other physical properties like a table. The rule is only used to populate the table, references in queries refer to the materialized data. This is a minimal implementation, but should still be useful in many cases. Currently data is only populated "on demand" by the CREATE MATERIALIZED VIEW and REFRESH MATERIALIZED VIEW statements. It is expected that future releases will add incremental updates with various timings, and that a more refined concept of defining what is "fresh" data will be developed. At some point it may even be possible to have queries use a materialized in place of references to underlying tables, but that requires the other above-mentioned features to be working first. Much of the documentation work by Robert Haas. Review by Noah Misch, Thom Brown, Robert Haas, Marko Tiikkaja Security review by KaiGai Kohei, with a decision on how best to implement sepgsql still pending.
2013-01-01Update copyrights for 2013Bruce Momjian
Fully update git head, and update back branches in ./COPYRIGHT and legal.sgml files.
2012-11-28Fix assorted bugs in CREATE/DROP INDEX CONCURRENTLY.Tom Lane
Commit 8cb53654dbdb4c386369eb988062d0bbb6de725e, which introduced DROP INDEX CONCURRENTLY, managed to break CREATE INDEX CONCURRENTLY via a poor choice of catalog state representation. The pg_index state for an index that's reached the final pre-drop stage was the same as the state for an index just created by CREATE INDEX CONCURRENTLY. This meant that the (necessary) change to make RelationGetIndexList ignore about-to-die indexes also made it ignore freshly-created indexes; which is catastrophic because the latter do need to be considered in HOT-safety decisions. Failure to do so leads to incorrect index entries and subsequently wrong results from queries depending on the concurrently-created index. To fix, add an additional boolean column "indislive" to pg_index, so that the freshly-created and about-to-die states can be distinguished. (This change obviously is only possible in HEAD. This patch will need to be back-patched, but in 9.2 we'll use a kluge consisting of overloading the formerly-impossible state of indisvalid = true and indisready = false.) In addition, change CREATE/DROP INDEX CONCURRENTLY so that the pg_index flag changes they make without exclusive lock on the index are made via heap_inplace_update() rather than a normal transactional update. The latter is not very safe because moving the pg_index tuple could result in concurrent SnapshotNow scans finding it twice or not at all, thus possibly resulting in index corruption. This is a pre-existing bug in CREATE INDEX CONCURRENTLY, which was copied into the DROP code. In addition, fix various places in the code that ought to check to make sure that the indexes they are manipulating are valid and/or ready as appropriate. These represent bugs that have existed since 8.2, since a failed CREATE INDEX CONCURRENTLY could leave a corrupt or invalid index behind, and we ought not try to do anything that might fail with such an index. Also fix RelationReloadIndexInfo to ensure it copies all the pg_index columns that are allowed to change after initial creation. Previously we could have been left with stale values of some fields in an index relcache entry. It's not clear whether this actually had any user-visible consequences, but it's at least a bug waiting to happen. In addition, do some code and docs review for DROP INDEX CONCURRENTLY; some cosmetic code cleanup but mostly addition and revision of comments. This will need to be back-patched, but in a noticeably different form, so I'm committing it to HEAD before working on the back-patch. Problem reported by Amit Kapila, diagnosis by Pavan Deolassee, fix by Tom Lane and Andres Freund.