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Overview
Comment: | Update the header comment on analyze.c to describe the sqlite_stat4 table format. |
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Downloads: | Tarball | ZIP archive |
Timelines: | family | ancestors | descendants | both | sqlite_stat4 |
Files: | files | file ages | folders |
SHA1: |
4d97809d6b29809f12d753043bda1976 |
User & Date: | drh 2013-08-09 14:07:55.264 |
Context
2013-08-09
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19:04 | Fix a couple of typos in a comment in analyze.c. No code changes. (check-in: 5bcccb93df user: dan tags: sqlite_stat4) | |
14:07 | Update the header comment on analyze.c to describe the sqlite_stat4 table format. (check-in: 4d97809d6b user: drh tags: sqlite_stat4) | |
2013-08-08
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19:38 | Fix problems in estimating the number of rows visited by a range query using sqlite_stat4 data when the column subject to the range query is not the leftmost of the index. (check-in: 9228aaf54d user: dan tags: sqlite_stat4) | |
Changes
Changes to src/analyze.c.
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16 17 18 19 20 21 22 23 24 25 26 27 28 29 | ** to help it make better decisions about how to perform queries. ** ** The following system tables are or have been supported: ** ** CREATE TABLE sqlite_stat1(tbl, idx, stat); ** CREATE TABLE sqlite_stat2(tbl, idx, sampleno, sample); ** CREATE TABLE sqlite_stat3(tbl, idx, nEq, nLt, nDLt, sample); ** ** Additional tables might be added in future releases of SQLite. ** The sqlite_stat2 table is not created or used unless the SQLite version ** is between 3.6.18 and 3.7.8, inclusive, and unless SQLite is compiled ** with SQLITE_ENABLE_STAT2. The sqlite_stat2 table is deprecated. ** The sqlite_stat2 table is superseded by sqlite_stat3, which is only ** created and used by SQLite versions 3.7.9 and later and with | > | | > > > > > | > | 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 | ** to help it make better decisions about how to perform queries. ** ** The following system tables are or have been supported: ** ** CREATE TABLE sqlite_stat1(tbl, idx, stat); ** CREATE TABLE sqlite_stat2(tbl, idx, sampleno, sample); ** CREATE TABLE sqlite_stat3(tbl, idx, nEq, nLt, nDLt, sample); ** CREATE TABLE sqlite_stat4(tbl, idx, nEq, nLt, nDLt, sample); ** ** Additional tables might be added in future releases of SQLite. ** The sqlite_stat2 table is not created or used unless the SQLite version ** is between 3.6.18 and 3.7.8, inclusive, and unless SQLite is compiled ** with SQLITE_ENABLE_STAT2. The sqlite_stat2 table is deprecated. ** The sqlite_stat2 table is superseded by sqlite_stat3, which is only ** created and used by SQLite versions 3.7.9 and later and with ** SQLITE_ENABLE_STAT3 defined. The functionality of sqlite_stat3 ** is a superset of sqlite_stat2. The sqlite_stat4 is an enhanced ** version of sqlite_stat3 and is only available when compiled with ** SQLITE_ENABLE_STAT4 and in SQLite versions 3.8.0 and later. ** ** For most applications, sqlite_stat1 provides all the statisics required ** for the query planner to make good choices. ** ** Format of sqlite_stat1: ** ** There is normally one row per index, with the index identified by the ** name in the idx column. The tbl column is the name of the table to ** which the index belongs. In each such row, the stat column will be ** a string consisting of a list of integers. The first integer in this ** list is the number of rows in the index. (This is the same as the ** number of rows in the table, except for partial indices.) The second ** integer is the average number of rows in the index that have the same ** value in the first column of the index. The third integer is the average ** number of rows in the index that have the same value for the first two ** columns. The N-th integer (for N>1) is the average number of rows in ** the index which have the same value for the first N-1 columns. For ** a K-column index, there will be K+1 integers in the stat column. If ** the index is unique, then the last integer will be 1. |
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79 80 81 82 83 84 85 | ** The format for sqlite_stat2 is recorded here for legacy reference. This ** version of SQLite does not support sqlite_stat2. It neither reads nor ** writes the sqlite_stat2 table. This version of SQLite only supports ** sqlite_stat3. ** ** Format for sqlite_stat3: ** | | | > | > > > > > > | | > | | > | > > > > > > > | | | < < < < < < < < < | | > > > > > > > > > > | 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 | ** The format for sqlite_stat2 is recorded here for legacy reference. This ** version of SQLite does not support sqlite_stat2. It neither reads nor ** writes the sqlite_stat2 table. This version of SQLite only supports ** sqlite_stat3. ** ** Format for sqlite_stat3: ** ** The sqlite_stat3 format is a subset of sqlite_stat4. Hence, the ** sqlite_stat4 format will be described first. Further information ** about sqlite_stat3 follows the sqlite_stat4 description. ** ** Format for sqlite_stat4: ** ** As with sqlite_stat2, the sqlite_stat4 table contains histogram data ** to aid the query planner in choosing good indices based on the values ** that indexed columns are compared against in the WHERE clauses of ** queries. ** ** The sqlite_stat4 table contains multiple entries for each index. ** The idx column names the index and the tbl column is the table of the ** index. If the idx and tbl columns are the same, then the sample is ** of the INTEGER PRIMARY KEY. The sample column is a blob which is the ** binary encoding of a key from the index, with the trailing rowid ** omitted. The nEq column is a list of integers. The first integer ** is the approximate number of entires in the index whose left-most ** column exactly matches the left-most column the sample. The second ** integer in nEq is the approximate number of entires in the index where ** the first two columns match the first two columns of the sample. ** And so forth. nLt is another list of integer that show the approximate ** number of entires that are strictly less than the sample. The first ** integer in nLt contains the number of entries in the index where the ** left-most column is less than the left-most column of the sample. ** The K-th integer in the nLt entry is the number of index entries ** where the first K columns are less than the first K columns of the ** sample. The nDLt column is like nLt except that it contains the ** number of distinct entries in the index that are less than the ** sample. ** ** There can be an arbitrary number of sqlite_stat4 entries per index. ** The ANALYZE command will typically generate sqlite_stat3 tables ** that contain between 10 and 40 samples which are distributed across ** the key space, though not uniformly, and which include samples with ** large nEq values. ** ** Format for sqlite_stat3 redux: ** ** The sqlite_stat3 table is like sqlite_stat4 except that it only ** looks at the left-most column of the index. The sqlite_stat3.sample ** column contains the actual value of the left-most column instead ** of a blob encoding of the complete index key as is found in ** sqlite_stat4.sample. The nEq, nLt, and nDLt entries of sqlite_stat3 ** all contain just a single integer which is the same as the first ** integer in the equivalent columns in sqlite_stat4. */ #ifndef SQLITE_OMIT_ANALYZE #include "sqliteInt.h" /* ** This routine generates code that opens the sqlite_stat1 table for ** writing with cursor iStatCur. If the library was built with the |
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