Clarification on the meaning of SrcList and SrcItem and especially the

SrcItem.colUsed and Index.colNotIdxed fields.  Comment changes only - no
changes to code.

FossilOrigin-Name: d96f6cc8475ae5509b8bff2db75e3c6f69a214d58d8979fbc0162ae488a040dc

manifest

@@ -1,5 +1,5 @@
-C Add\sSQLITE_DQS\sto\sthe\scompileoptions_used\slist,\sper\srequest\sin\s[forum\spost\s8b1060122b|forum:8b1060122b].\sForce\sDQS=0\sin\ssqlite3-wasm.c.
+C Clarification\son\sthe\smeaning\sof\sSrcList\sand\sSrcItem\sand\sespecially\sthe\nSrcItem.colUsed\sand\sIndex.colNotIdxed\sfields.\s\sComment\schanges\sonly\s-\sno\nchanges\sto\scode.
-D 2022-10-21T17:48:49.786
+D 2022-10-22T13:49:35.916
 F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
 F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
 F LICENSE.md df5091916dbb40e6e9686186587125e1b2ff51f022cc334e886c19a0e9982724
@@ -562,7 +562,7 @@ F src/btmutex.c 6ffb0a22c19e2f9110be0964d0731d2ef1c67b5f7fabfbaeb7b9dabc4b7740ca
 F src/btree.c 74fc5f6a0577df703d6f98d0c51ee0d8d91d22dbc0ba86e42e056517e2b45576
 F src/btree.h 74d64b8f28cfa4a894d14d4ed64fa432cd697b98b61708d4351482ae15913e22
 F src/btreeInt.h 8ce1332edd89dfd2461d561ac10a0ab5601c8e06200cb5230596c3caaf54482e
-F src/build.c 13bcc04821ad260ef00d87e0704424e0ae4e6cd66c7e384b4cedb379a6cb738b
+F src/build.c d3e43e950e4e377c1d451a4862556792acdef1faba14a03f899d30d09731c48b
 F src/callback.c 4cd7225b26a97f7de5fee5ae10464bed5a78f2adefe19534cc2095b3a8ca484a
 F src/complete.c a3634ab1e687055cd002e11b8f43eb75c17da23e
 F src/ctime.c 20507cc0b0a6c19cd882fcd0eaeda32ae6a4229fb4b024cfdf3183043d9b703d
@@ -625,7 +625,7 @@ F src/shell.c.in 6a9e15cb9fc3cd13d3647d4d9714c0d4d4a65e7f49228c2aafca910ed08d577
 F src/sqlite.h.in d9c8a6243fc0a1c270d69db33758e34b810af3462f9bc5b4af113b347e07c69d
 F src/sqlite3.rc 5121c9e10c3964d5755191c80dd1180c122fc3a8
 F src/sqlite3ext.h 5336beea1868d99d2f62e628dbea55e97267dbff8193291ab175e960c5df9141
-F src/sqliteInt.h ce8bf543210ab4a8a797e6abaf45e15f4cff24570b1043560aa747b3dc1a7935
+F src/sqliteInt.h 36f456f599a1bda4287ece61f3a9305b0b8d90ff49017a47a981cda5721a7ce0
 F src/sqliteLimit.h d7323ffea5208c6af2734574bae933ca8ed2ab728083caa117c9738581a31657
 F src/status.c 160c445d7d28c984a0eae38c144f6419311ed3eace59b44ac6dafc20db4af749
 F src/table.c 0f141b58a16de7e2fbe81c308379e7279f4c6b50eb08efeec5892794a0ba30d1
@@ -2036,8 +2036,8 @@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a91a7acc6293828957a386a8a93
 F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
 F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
 F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
-P 9cf1142b0cdb13347e3f551c862cb4714cadfe5ad637f049cf0a4b8bb6125b32
+P fcd9e0dbe3226f3f7ccc15b11fc3aa3b8058571bef274c25a33e9753e22f7551
-R f0d8c3eadf9b3a16756631ad00044092
+R 3bee91f5b3c5cc3b42735acd38a39745
-U stephan
+U drh
-Z 4b9ac72991a7e1ed1b8b1873a504bb5c
+Z 7715147e0f497c16dde985e20f495b12
 # Remove this line to create a well-formed Fossil manifest.

manifest.uuid

@@ -1 +1 @@
-fcd9e0dbe3226f3f7ccc15b11fc3aa3b8058571bef274c25a33e9753e22f7551
+d96f6cc8475ae5509b8bff2db75e3c6f69a214d58d8979fbc0162ae488a040dc

src/build.c

@@ -2276,7 +2276,8 @@ static int isDupColumn(Index *pIdx, int nKey, Index *pPk, int iCol){
 /* Recompute the colNotIdxed field of the Index.
 **
 ** colNotIdxed is a bitmask that has a 0 bit representing each indexed
-** columns that are within the first 63 columns of the table.  The
+** columns that are within the first 63 columns of the table and a 1 for
+** all other bits (all columns that are not in the index).  The
 ** high-order bit of colNotIdxed is always 1.  All unindexed columns
 ** of the table have a 1.
 **
@@ -2304,7 +2305,7 @@ static void recomputeColumnsNotIndexed(Index *pIdx){
     }
   }
   pIdx->colNotIdxed = ~m;
-  assert( (pIdx->colNotIdxed>>63)==1 );
+  assert( (pIdx->colNotIdxed>>63)==1 );  /* See note-20221022-a */
 }
 
 /*

src/sqliteInt.h

@@ -2579,10 +2579,22 @@ struct UnpackedRecord {
 ** The Index.onError field determines whether or not the indexed columns
 ** must be unique and what to do if they are not.  When Index.onError=OE_None,
 ** it means this is not a unique index.  Otherwise it is a unique index
-** and the value of Index.onError indicate the which conflict resolution
+** and the value of Index.onError indicates which conflict resolution
-** algorithm to employ whenever an attempt is made to insert a non-unique
+** algorithm to employ when an attempt is made to insert a non-unique
 ** element.
 **
+** The colNotIdxed bitmask is used in combination with SrcItem.colUsed
+** for a fast test to see if an index can serve as a covering index.
+** colNotIdxed has a 1 bit for every column of the original table that
+** is *not* available in the index.  Thus the expression
+** "colUsed & colNotIdxed" will be non-zero if the index is not a
+** covering index.  The most significant bit of of colNotIdxed will always
+** be true (note-20221022-a).  If a column beyond the 63rd column of the
+** table is used, the "colUsed & colNotIdxed" test will always be non-zero
+** and we have to assume either that the index is not covering, or use
+** an alternative (slower) algorithm to determine whether or not
+** the index is covering.
+**
 ** While parsing a CREATE TABLE or CREATE INDEX statement in order to
 ** generate VDBE code (as opposed to parsing one read from an sqlite_schema
 ** table as part of parsing an existing database schema), transient instances
@@ -2628,7 +2640,7 @@ struct Index {
   tRowcnt *aiRowEst;       /* Non-logarithmic stat1 data for this index */
   tRowcnt nRowEst0;        /* Non-logarithmic number of rows in the index */
 #endif
-  Bitmask colNotIdxed;     /* 0 for unindexed columns in pTab */
+  Bitmask colNotIdxed;     /* Unindexed columns in pTab */
 };
 
 /*
@@ -3081,6 +3093,14 @@ struct IdList {
 ** The SrcItem object represents a single term in the FROM clause of a query.
 ** The SrcList object is mostly an array of SrcItems.
 **
+** The jointype starts out showing the join type between the current table
+** and the next table on the list.  The parser builds the list this way.
+** But sqlite3SrcListShiftJoinType() later shifts the jointypes so that each
+** jointype expresses the join between the table and the previous table.
+**
+** In the colUsed field, the high-order bit (bit 63) is set if the table
+** contains more than 63 columns and the 64-th or later column is used.
+**
 ** Union member validity:
 **
 **    u1.zIndexedBy          fg.isIndexedBy && !fg.isTabFunc
@@ -3120,7 +3140,7 @@ struct SrcItem {
     Expr *pOn;        /* fg.isUsing==0 =>  The ON clause of a join */
     IdList *pUsing;   /* fg.isUsing==1 =>  The USING clause of a join */
   } u3;
-  Bitmask colUsed;  /* Bit N (1<<N) set if column N of pTab is used */
+  Bitmask colUsed;  /* Bit N set if column N used. Details above for N>62 */
   union {
     char *zIndexedBy;    /* Identifier from "INDEXED BY <zIndex>" clause */
     ExprList *pFuncArg;  /* Arguments to table-valued-function */
@@ -3141,23 +3161,11 @@ struct OnOrUsing {
 };
 
 /*
-** The following structure describes the FROM clause of a SELECT statement.
+** This object represents one or more tables that are the source of
-** Each table or subquery in the FROM clause is a separate element of
+** content for an SQL statement.  For example, a single SrcList object
-** the SrcList.a[] array.
+** is used to hold the FROM clause of a SELECT statement.  SrcList also
+** represents the target tables for DELETE, INSERT, and UPDATE statements.
 **
-** With the addition of multiple database support, the following structure
-** can also be used to describe a particular table such as the table that
-** is modified by an INSERT, DELETE, or UPDATE statement.  In standard SQL,
-** such a table must be a simple name: ID.  But in SQLite, the table can
-** now be identified by a database name, a dot, then the table name: ID.ID.
-**
-** The jointype starts out showing the join type between the current table
-** and the next table on the list.  The parser builds the list this way.
-** But sqlite3SrcListShiftJoinType() later shifts the jointypes so that each
-** jointype expresses the join between the table and the previous table.
-**
-** In the colUsed field, the high-order bit (bit 63) is set if the table
-** contains more than 63 columns and the 64-th or later column is used.
 */
 struct SrcList {
   int nSrc;        /* Number of tables or subqueries in the FROM clause */